Tính:
a,\(A=\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\)
b,\(B=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
c,\(C=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{1989.1990}+...+\frac{1}{2006.2007}\)
Những câu hỏi liên quan
Tính nhanh :
a)\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}...+\frac{1}{98.99}+\frac{1}{99.100}\)
b)\(\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{41.43}\)
c)\(\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\)
d)\(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{90}+\frac{1}{110}\)
\(\frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+...+\frac{1}{99\cdot100}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)
\(=1-\frac{1}{100}\)
\(=\frac{99}{100}\)
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Bài 1: Cho số text{A 123456789101112.........585960}.
a) Số A có bao nhiêu chữ số ?
b) Hãu xóa đi 100 chữ số trong A sao cho số còn lại là:
+ Nhỏ nhất + Lớn nhất
Bài 2: Tính :
a) frac{5}{11.16}+frac{5}{16.21}+frac{5}{21.26}+...+frac{5}{61.66}
b) frac{1}{2}+frac{1}{6}+frac{1}{12}+frac{1}{20}+frac{1}{30}+frac{1}{42}
c) frac{1}{1.2}+frac{1}{2.3}+...+frac{1}{1989.1990}+...+frac{1}{2006.2007}
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Bài 1: Cho số \(\text{A = 123456789101112.........585960}.\)
a) Số A có bao nhiêu chữ số ?
b) Hãu xóa đi 100 chữ số trong A sao cho số còn lại là:
+ Nhỏ nhất + Lớn nhất
Bài 2: Tính :
a) \(\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\)
b) \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
c) \(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{1989.1990}+...+\frac{1}{2006.2007}\)
Bài 2:
a) \(\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\)
\(=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)
\(=\frac{1}{11}-\frac{1}{66}\)
\(=\frac{5}{66}\)
b) \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{30}+\frac{1}{42}\)
\(=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
\(=1-\frac{1}{7}\)
\(=\frac{6}{7}\)
c) \(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{2006.2007}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{2006}-\frac{1}{2007}\)
\(=1-\frac{1}{2007}\)
\(=\frac{2006}{2007}\)
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Bài 2:
a) \(\frac{5}{11.16}\) + \(\frac{5}{16.21}\) + \(\frac{5}{21.26}\) + ... + \(\frac{5}{61.66}\)
= \(\frac{1}{11}\) - \(\frac{1}{16}\) + \(\frac{1}{16}\) - \(\frac{1}{21}\) + \(\frac{1}{21}\) - \(\frac{1}{26}\) + ... + \(\frac{1}{61}\) - \(\frac{1}{66}\)
= \(\frac{1}{11}\) - \(\frac{1}{66}\)
= \(\frac{5}{66}\)
b) \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
= \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
= \(1-\frac{1}{7}\)
= \(\frac{6}{7}\)
c) \(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{1989.1990}+...+\frac{1}{2006.2007}\)
= \(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{1989}-\frac{1}{1990}+...+\frac{1}{2006}-\frac{1}{2007}\)
= \(1-\frac{1}{2007}\)
= \(\frac{2006}{2007}\)
Chúc bạn học tốt!
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Bài 1: a) Afrac{5}{11.16}+frac{5}{16.21}+frac{5}{21.26}+...+frac{5}{61.66} b) Bfrac{1}{2}+frac{1}{6}+frac{1}{12}+frac{1}{20}+frac{1}{30}+frac{1}{42} c) Cfrac{1}{1.2}+frac{1}{2.3}+...+frac{1}{1989.1990}Bài 2: a. Tính tổng: Mfrac{10}{56}+frac{10}{140}+frac{10}{260}+...+frac{10}{1400} b. Cho: Sfrac{3}{10}+frac{3}{11}+frac{3}{12}+frac{3}{13}+frac{3}{14} chứng minh rằng 1 S 2Bài 3: Tính giá trị của biểu thức sau:Aleft(frac{1}{7}+frac{1}{23}-frac{1}{1009}right):left(frac{1...
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Bài 1: a) \(A=\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\)
b) \(B=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
c) \(C=\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{1989.1990}\)
Bài 2: a. Tính tổng: \(M=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
b. Cho: \(S=\frac{3}{10}+\frac{3}{11}+\frac{3}{12}+\frac{3}{13}+\frac{3}{14}\) chứng minh rằng 1 < S < 2
Bài 3: Tính giá trị của biểu thức sau:
\(A=\left(\frac{1}{7}+\frac{1}{23}-\frac{1}{1009}\right):\left(\frac{1}{23}+\frac{1}{7}-\frac{2}{2009}+\frac{1}{7}.\frac{1}{23}.\frac{1}{2009}\right)+1:\left(30.1009-160\right)\)
Bài 4: Tính nhanh:
\(\text{a) 35 . 34 + 35 . 86 + 67 . 75 + 65 . 45}\)
\(\text{b) 21 . }7^2-11.7^2+90.7^2+49.125.16\)
Bài 5: Thực hiện phép tinh sau:
a. \(\frac{2181.729+243.81.27}{3^2.9^2.234+18.54+162.9+723.729}\)
b. \(\frac{1}{1.2+}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{98.99}+\frac{1}{99.100}\)
c. \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{100^2}< 1\)
d. \(\frac{5.4^{15}-9^9-4.3^{20}}{5.2^{19}.6^{19}-7.2^{29}.27^6}\)
giúp mk nha! nhớ viết cách làm nha!
Bài 1 mik học xong quên hết òi (mấy bài kia là hok biết luôn :V)
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A=\(\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+....+\frac{5}{61.66}\)A=\(\frac{5}{11}-\frac{5}{16}+\frac{5}{16}-\frac{5}{21}+...+\frac{5}{61}-\frac{5}{66}\)A=5/11-5/66A=25/66
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B= \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
B=\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}\)
B=1-\(\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}\)
B=1-1/7
B=6/7
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Tính các tổng sau:a) A frac{1}{6}+ frac{1}{12}+ frac{1}{20}+ frac{1}{30}+ frac{1}{42} + frac{1}{56}b) B frac{5}{11.16}+ frac{5}{16.21}+ .....+frac{5}{61.66}Các bạn làm nhanh giúp mình nhé
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Tính các tổng sau:
a) A = \(\frac{1}{6}\)+ \(\frac{1}{12}\)+ \(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\) + \(\frac{1}{56}\)
b) B= \(\frac{5}{11.16}\)+ \(\frac{5}{16.21}\)+ .....+\(\frac{5}{61.66}\)
Các bạn làm nhanh giúp mình nhé
a/ \(A=\frac{1}{6}+\frac{1}{12}+.........+\frac{1}{56}\)
\(=\frac{1}{2.3}+\frac{1}{3.4}+..........+\frac{1}{7.8}\)
\(=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+.........+\frac{1}{7}-\frac{1}{8}\)
\(=\frac{1}{2}-\frac{1}{8}=\frac{3}{4}\)
b/ \(B=\frac{5}{11.16}+\frac{5}{16.21}+........+\frac{5}{61.66}\)
\(=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+........+\frac{1}{61}-\frac{1}{66}\)
\(=\frac{1}{11}-\frac{1}{66}\)
\(=\frac{5}{66}\)
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a) \(A=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(A=\frac{1}{2}-\frac{1}{8}=\frac{3}{8}\)
b) \(B=\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\)
\(B=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)
\(B=\frac{1}{11}-\frac{1}{66}=\frac{5}{66}\)
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a) \(A=\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)
\(A=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)
\(A=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)
\(A=\frac{1}{2}-\frac{1}{8}\)
\(A=\frac{3}{8}\)
b) \(B=\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\)
\(5B=5\left(\frac{1}{11.16}+\frac{1}{16.21}+...+\frac{1}{61.66}\right)\)
\(\Rightarrow B=\frac{1}{11.16}+\frac{1}{16.21}+...+\frac{1}{61.66}\)
\(B=\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)
\(B=\frac{1}{11}-\frac{1}{66}\)
\(B=\frac{5}{66}\)
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Tính hợp lý a, 16frac{3}{31}-left(frac{19}{28}+frac{13}{31}right)b, left(24frac{5}{13}+7frac{9}{46}right)-31frac{5}{13}c, frac{1}{20}+frac{1}{30}+frac{1}{42}+frac{1}{56}+frac{1}{72}+frac{1}{90}+frac{1}{110}d,frac{5^2}{1.6}+frac{5^2}{6.11}+frac{5^2}{11.16}+frac{5^2}{16.21}+frac{5^2}{21.26}+frac{5^2}{26.31}e, 1.2+2.4+3.5+4.6+5.7+.............+98.101f,left(frac{1}{2^2}-1right).left(frac{1}{3}-1right).left(frac{1}{4}-1right).............left(frac{1}{100^2}-1right) nói cách làm nhé ,làm dc 1 câu cũng...
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Tính hợp lý
a, \(16\frac{3}{31}-\left(\frac{19}{28}+\frac{13}{31}\right)\)
b, \(\left(24\frac{5}{13}+7\frac{9}{46}\right)-31\frac{5}{13}\)
c, \(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}\)
d,\(\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}+\frac{5^2}{26.31}\)
e, 1.2+2.4+3.5+4.6+5.7+.............+98.101
f,\(\left(\frac{1}{2^2}-1\right).\left(\frac{1}{3}-1\right).\left(\frac{1}{4}-1\right).............\left(\frac{1}{100^2}-1\right)\)
nói cách làm nhé ,làm dc 1 câu cũng được ^-^
a,b you cứ tính bt nhé
c)\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{110}\)
\(=\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}+\frac{1}{10.11}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{10}-\frac{1}{11}\)
\(=\frac{1}{4}-\frac{1}{11}\)
\(=\frac{7}{44}\)
d) \(\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}+\frac{5^2}{26.31}\)
\(=5\left(\frac{5}{1.6}+\frac{5}{6.11}+\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+\frac{5}{26.31}\right)\)
\(=5\left(1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+\frac{1}{26}-\frac{1}{31}\right)\)
\(=5\left(1-\frac{1}{31}\right)\)
\(=5.\frac{30}{31}\)
\(=\frac{150}{31}\)
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A = \(\frac{\frac{1}{11}-\frac{1}{13}-\frac{1}{17}}{\frac{5}{11}-\frac{5}{13}-\frac{5}{17}}+\frac{\frac{2}{3}-\frac{2}{9}-\frac{2}{27}+\frac{2}{81}}{\frac{7}{3}-\frac{7}{9}-\frac{7}{27}+\frac{7}{81}}\)
B = \(\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}+\frac{5^2}{26.31}+...+\frac{5^2}{56.81}\)
C = \(1-\frac{1}{3}-\frac{1}{6}-\frac{1}{10}-\frac{1}{15}-...-\frac{1}{1225}\)
a)
\(\Rightarrow A=\frac{\frac{1}{11}-\frac{1}{13}-\frac{1}{17}}{5\left(\frac{1}{11}-\frac{1}{13}-\frac{1}{17}\right)}+\frac{2\left(\frac{1}{3}-\frac{1}{9}-\frac{1}{27}+\frac{1}{81}\right)}{7\left(\frac{1}{3}-\frac{1}{9}-\frac{1}{27}+\frac{1}{81}\right)}\)
\(\Rightarrow A=\frac{1}{5}+\frac{2}{7}\)
\(\Rightarrow A=\frac{17}{35}\)
b)
\(\Rightarrow B=5\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+....+\frac{1}{56}-\frac{1}{61}\right)\)
\(\Rightarrow B=5\left(\frac{1}{11}-\frac{1}{61}\right)\)
\(\Rightarrow B=5.\frac{50}{671}=\frac{250}{671}\)
c)
\(\Rightarrow C=1-\left(\frac{1}{1.3}+\frac{1}{2.3}+\frac{1}{2.5}+....+\frac{1}{49.25}\right)\)
\(\Rightarrow C=1-2\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+....+\frac{1}{49.50}\right)\)
\(\Rightarrow C=1-2\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{49}-\frac{1}{50}\right)\)
\(\Rightarrow C=1-1-\frac{1}{25}\)
\(\Rightarrow C=\frac{1}{25}\)
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tính nhanh: \(\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}+...+\frac{1}{61.66}\)
\(\frac{1}{11.16}+\frac{1}{16.21}+\frac{1}{21.26}+...+\frac{1}{61.66}\)
=\(\frac{1}{5}.\frac{5}{11.16}+\frac{1}{5}.\frac{5}{16.21}+\frac{1}{5}.\frac{5}{21.26}+...+\frac{1}{5}.\frac{5}{61.66}\)
=\(\frac{1}{5}.\left(\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+...+\frac{5}{61.66}\right)\)
=\(\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)
=\(\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{66}\right)\)
=\(\frac{1}{5}.\left(\frac{6}{66}-\frac{1}{66}\right)=\frac{1}{5}.\frac{5}{66}=\frac{1}{66}\)
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Đặt A = \(\frac{1}{11.16}+...+\frac{1}{61.66}\)
5A = \(\frac{5}{11.16}+..+\frac{5}{61.66}\)
5a = \(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\)
5a = \(\frac{1}{11}-\frac{1}{61}\)
5a = 50/671
a = \(\frac{50}{671}:5=\frac{10}{671}\)
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\(\frac{1}{11.16}+\frac{1}{16.21}\)\(+\frac{1}{21.26}+...+\frac{1}{61.66}\)
= \(\frac{1}{5}.\left(\frac{5}{11.16}+\frac{5}{16.21}+...+\frac{5}{61.66}\right)\)
= \(\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+...+\frac{1}{61}-\frac{1}{66}\right)\)
= \(\frac{1}{5}.\left(\frac{1}{11}-\frac{1}{66}\right)\)
= \(\frac{1}{5}.\left(\frac{5}{6}\right)\)
=\(\frac{1}{6}\)
k mk nha !
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Đây là câu trả lời của Đinh Nguyễn Nguyệt HàAfrac{5.2}{1.6}+frac{5.2}{6.11}+frac{5.2}{11.16}+frac{5.2}{16.21}+frac{5.2}{21.26}+frac{5.2}{26.31}frac{A}{2}frac{5}{1.6}+frac{5}{6.11}+frac{5}{11.16}+frac{5}{16.21}+frac{5}{21.26}+frac{5}{26.31} 1-frac{1}{6}+frac{1}{6}-frac{1}{11}+frac{1}{11}-frac{1}{16}+frac{1}{16}-frac{1}{21}+frac{1}{21}-frac{1}{26}+frac{1}{26}-frac{1}{31} 1-frac{1}{31}frac{30}{31}Rightarrow Afrac{30}{31}.2frac{60}{31}Nếu tử là 52 thì cậu chuyển thành 5.5 rồi chuyển 5 sang v...
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Đây là câu trả lời của Đinh Nguyễn Nguyệt Hà
\(A=\frac{5.2}{1.6}+\frac{5.2}{6.11}+\frac{5.2}{11.16}+\frac{5.2}{16.21}+\frac{5.2}{21.26}+\frac{5.2}{26.31}\)
\(\frac{A}{2}=\frac{5}{1.6}+\frac{5}{6.11}+\frac{5}{11.16}+\frac{5}{16.21}+\frac{5}{21.26}+\frac{5}{26.31}\)
\(=1-\frac{1}{6}+\frac{1}{6}-\frac{1}{11}+\frac{1}{11}-\frac{1}{16}+\frac{1}{16}-\frac{1}{21}+\frac{1}{21}-\frac{1}{26}+\frac{1}{26}-\frac{1}{31}\)
\(=1-\frac{1}{31}=\frac{30}{31}\)
\(\Rightarrow A=\frac{30}{31}.2=\frac{60}{31}\)
Nếu tử là 52 thì cậu chuyển thành 5.5 rồi chuyển 5 sang vế trái thành A/5 nha, cũng giống như trên thui!!!!!!!!
tính hợp lí
a) 1+6+11+16+.......+46+51
b) \(\frac{5^2}{1.6}+\frac{5^2}{6.11}+\frac{5^2}{11.16}+\frac{5^2}{16.21}+\frac{5^2}{21.26}+\frac{5^2}{26.31}\)
a) áp dụng dãy số cách đều đi
a, 1+6+11+16+...+46+51
Số số hạng là : (51-1):5+1 = 11 ( số )
Tổng là : (51+1).11:2=286
b, Đặt A = \(\dfrac{5^2}{1.6}+\dfrac{5^2}{6.11}+\dfrac{5^2}{11.16}+\dfrac{5^2}{16.21}+\dfrac{5^2}{21.26}+\dfrac{5^2}{26.31 } \)
\(\dfrac{1}{5}A=\) \(\dfrac{5}{1.6}+\dfrac{5}{6.11}+\dfrac{5}{11.16}+\dfrac{5}{16.21}+\dfrac{5}{21.26}+\dfrac{5}{26.31}\)
\(\dfrac{1}{5}A=\) \(1-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{16}+\dfrac{1}{16}-\dfrac{1}{21}+\dfrac{1}{21}-\dfrac{1}{26}+\dfrac{1}{26}-\dfrac{1}{31}\)
\(\dfrac{1}{5}A=1-\dfrac{1}{31}\)
\(\dfrac{1}{5}A=\dfrac{30}{31}\)
\(A=\dfrac{30}{31}:\dfrac{1}{5}=\dfrac{150}{31}\)
Vậy..