1/20+1/30+1/42+1/56+1/72+1/90 = ?
Tính: B=1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72
A=1/90-1/72-1/56-1/42-1/30-1/20-1/12-1/6-1/2
em lớp 6 nha
B= 1/2 + 1/6 + 1/12 +1/20 + 1/30 + 1/42 + 1/56 + 1/72
B= 1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 + 1/5*6 + 1/6*7 + 1/7*8 + 1/8*9
B=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9
B=1+0-0-0-0-0-0-0-1/9
B=1-1/9
B=8/9
k em nha
-1/20+ -1/30+ -1/42+ -1/56+ -1/72+ -1/90=?
(-1/20)+(-1/30)+(-1/42)+(-1/56)+(-1/72)+(-1/90)
(-1/20)+(-1/30)+(-1/42)+(-1/56)+(-1/72)+(-1/90)= - 3/20
\(\frac{-1}{20}+\frac{-1}{30}+\frac{-1}{42}+\frac{-1}{56}+\frac{-1}{72}+\frac{-1}{90}\)
= \(\frac{-1}{4.5}+\frac{-1}{5.6}+\frac{-1}{6.7}+\frac{-1}{7.8}+\frac{-1}{8.9}+\frac{-1}{9.10}\)
= \(\left(-1\right).\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}+\frac{1}{8.9}+\frac{1}{9.10}\right)\)
= \(\left(-1\right).\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}+\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}\right)\)
= \(\left(-1\right).\left(\frac{1}{4}-\frac{1}{10}\right)\)
= \(\left(-1\right).\frac{3}{20}\)
= \(\frac{-3}{20}\)
-1/20+ -1/30+ -1/42+ -1/56+ -1/72+ -1/90
đặt dấu âm ra ngoài
=-(1/20+1/30+1/42+..........1/90)
=-(1/4.5+1/5.6+1/6.7+......1/9.10)
=-(1/4-1/5+1/5-1/6+.....+1/9-1/10)
=-(1/4-1/10)
=-0,15
1/20+ 1/30+ 1/42+ 1/56+ 1/72+1/90
6323121356214488888888888888888888888888888888888888888888888888888888888888888888
\(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)
\(=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)
\(=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{4}-\frac{1}{10}=\frac{5}{20}-\frac{2}{20}=\frac{3}{20}\)
\(\frac{1}{20}\)+ \(\frac{1}{30}\)+ \(\frac{1}{42}\)+ \(\frac{1}{56}\)+ \(\frac{1}{72}\)+ \(\frac{1}{90}\)
= \(\frac{1}{4.5}\)+ \(\frac{1}{5.6}\)+ \(\frac{1}{6.7}\)+ \(\frac{1}{7.8}\)+ \(\frac{1}{8.9}\)+ \(\frac{1}{9.10}\)
= \(\frac{1}{4}\)- \(\frac{1}{5}\)+ \(\frac{1}{5}\)- \(\frac{1}{6}\)+ .... + \(\frac{1}{9}\)- \(\frac{1}{10}\)
= \(\frac{1}{4}\)- \(\frac{1}{10}\)
= \(\frac{5}{20}\)- \(\frac{2}{20}\)
= \(\frac{3}{20}\)
-1/20+ -1/30+ -1/42+ -1/56+ -1/72+ -1/90=?
-1/20=-1/4*5, -1/30=-1/5*6, -1/42=-1/6*7, -1/56=-1/7*8, -1/72=-1/8*9, -1/90=9*10
=> -1/4*5+-1/5*6 + -1/6*7 + -1/7*8 + -1/8*9 + -1/9*10=-1/4- -1/5 + ..........= -1/4 - -1/10 = -3/20
Tính tổng : A = -1/20 +-1/30 +-1/42 + -1/56+ -1/72 + -1/90
\(A=-\dfrac{1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\\ A=-\left(\dfrac{1}{4.5}+\dfrac{1}{5.6}+\dfrac{1}{6.7}+\dfrac{1}{7.8}+\dfrac{1}{8.9}+\dfrac{1}{9.10}\right)\\ A=-\left(\dfrac{5-4}{4.5}+\dfrac{6-5}{5.6}+\dfrac{7-6}{6.7}+\dfrac{8-7}{7.8}+\dfrac{9-8}{8.9}+\dfrac{10-9}{9.10}\right)\\ A=-\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\right)\\ A=-\left(\dfrac{1}{4}-\dfrac{1}{10}\right)=-\dfrac{3}{20}.\)
A = 1/20 +1/30 +1/42 +1/56 +1/72 + 1/90
\(A=\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)
\(A=\frac{1}{4\cdot5}+\frac{1}{5\cdot6}+\frac{1}{6\cdot7}+\frac{1}{7\cdot8}+\frac{1}{8\cdot9}+\frac{1}{9\cdot10}\)
\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-...+\frac{1}{9}-\frac{1}{10}\)
\(A=\frac{1}{4}-\frac{1}{10}\)
\(A=\frac{3}{20}\)
\(A=\frac{1}{4.5}+\frac{1}{5.6}+...+\frac{1}{9.10}\)
\(A=\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+...+\frac{1}{9}-\frac{1}{10}\)
\(A=\frac{1}{4}-\frac{1}{10}=\frac{5}{20}-\frac{2}{20}=\frac{3}{20}\)
A = \(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)
A=
1/20+1/30+1/42+1/56+1/72+1/90+... TÍNH.?
Đến bao nhiêu, bạn theo cách tính mà ra kết quả cuối
T = 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132
T = 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 + 1/10.11 + 1/11.12
T = 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 + 1/11 - 1/12
T = 1/4 - 1/12 (Cứ hai thằng cạnh nhau cộng lại bằng 0, chỉ còn thằng đầu và thằng cuối)
T = (3 - 1)/12
T = 2/12
T = 1/6
T = 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 + 1/132
T = 1/4.5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10 + 1/10.11 + 1/11.12
T = 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 + 1/11 - 1/12
T = 1/4 - 1/12 (Cứ hai thằng cạnh nhau cộng lại bằng 0, chỉ còn thằng đầu và thằng cuối)
T = (3 - 1)/12
T = 2/12
T = 1/6
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