Tính nhanh: 81x56/7x9=......
tính nhanh
3332 : [ 4x49]
b 72x3x125
c 123 :1,3+123:7,8+123:0,9
d 15x24+15x57+81x29+81x56
a, \(3332:\left(4.49\right)=3332:196=17\)
b, \(72.3.125=\left(72.125\right).3=9000.3=27000\)
c, \(123:1,3+123:7,8+123:0,9=123:\left(1,3+7,8+0,9\right)=123.10=1230\)
d, \(15.24+15.57+81.29+81.56=15\left(24+57\right)+81\left(29+56\right)=15.81+81.85=81\left(15+85\right)=81.100=8100\)
tính nhanh 6 / 5x7 + 6 / 7x9 + ....... + 6 / 59x61
\(\dfrac{6}{5.7}+\dfrac{6}{7.9}+...+\dfrac{6}{59.61}\)
\(=3\left(\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{59.61}\right)\)
\(=3\left(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{59}-\dfrac{1}{61}\right)\)
\(=3\left(\dfrac{1}{5}-\dfrac{1}{61}\right)\)
\(=\dfrac{3.56}{305}\\ =\dfrac{168}{305}\)
\(=3\left(\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{59}-\dfrac{1}{61}\right)=3\cdot\left(\dfrac{1}{5}-\dfrac{1}{61}\right)=3\cdot\dfrac{56}{305}=\dfrac{168}{305}\)
\(B=\dfrac{3}{3x5}+\dfrac{3}{5x7}+\dfrac{3}{7x9}+....+\dfrac{3}{48x50}\)Tính nhanh:
\(B=\dfrac{3}{3x5}+\dfrac{3}{5x7}+\dfrac{3}{7x9}+....+\dfrac{3}{48x50}\)
\(B=\dfrac{3}{3x5}+\dfrac{3}{5x7}+\dfrac{3}{7x9}+....+\dfrac{3}{48x50}\)
\(B=\dfrac{3}{3x5}+\dfrac{3}{5x7}+\dfrac{3}{7x9}+....+\dfrac{3}{48x50}\)
Giải:
\(B=\dfrac{3}{3\times5}+\dfrac{3}{5\times7}+\dfrac{3}{7\times9}+...+\dfrac{3}{48\times50}\)
\(B=\dfrac{3}{2}\times\left(\dfrac{2}{3\times5}+\dfrac{2}{5\times7}+\dfrac{2}{7\times9}+...+\dfrac{2}{48\times50}\right)\)
\(B=\dfrac{3}{2}\times\left(\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{9}+...+\dfrac{1}{48}-\dfrac{1}{50}\right)\)
\(B=\dfrac{3}{2}\times\left(\dfrac{1}{3}-\dfrac{1}{50}\right)\)
\(B=\dfrac{3}{2}\times\dfrac{47}{150}\)
\(B=\dfrac{47}{100}\)
Chúc em học tốt!
tính nhanh
1x3 + 3x5 + 5x7 + 7x9 + ......... + 97x99
Tính nhanh: S= 2/5x7 + 2/7x9 + 2/9x11 + ... + 2/93x95
S=1/5-1/7+1/7-1/9+1/9-1/11+...+1/93-1/95
S=1/5-1/95
S=18/95
a, 2/1x3 + 2/3x5 + 2/5x7 + 2/7x9 +...+ 9/913 x 215
b,1/1x3 + 1/3x5 + 1/5x7 + 1/7x9 + 1/213 x 215
[ Giúp mik với mấy bạn ơi ai nhanh mình sẽ tick nha TvT ]
( Toán tính nhanh nha )
sửa đề câu a và câu b nhá , mik nghĩ đề như này :
\(\frac{2}{1\cdot3}+\frac{2}{3\cdot5}+\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+...+\frac{2}{213\cdot215}\)
\(=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{213}-\frac{1}{215}\)
= \(\frac{1}{1}-\frac{1}{215}\)
\(=\frac{214}{215}\)
b, đặt \(A=\frac{1}{1\cdot3}+\frac{1}{3\cdot5}+\frac{1}{5\cdot7}+\frac{1}{7\cdot9}+...+\frac{1}{213\cdot215}\)
\(A\cdot2=\frac{2}{1\cdot3}+\frac{2}{3.5}+\frac{2}{5\cdot7}+\frac{2}{7\cdot9}+...+\frac{2}{213\cdot215}\)
\(A\cdot2=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{213}-\frac{1}{215}\)
\(A\cdot2=\frac{1}{1}-\frac{1}{215}\)
\(A\cdot2=\frac{214}{215}\)
\(A=\frac{214}{215}:2\)
\(A=\frac{107}{215}\)
@ミ★Ŧɦươйǥ★彡 cảm ơn bạn nhiều
trả lời hiền thương đề bài của bạn ấy là đúm gòi nha
tính nhanh biểu thức sau :
1/3x5 +1/5x7 +1/7x9 +1/9x11
ai làm nhanh , đúng mà phải tính nhanh thì mình mới tk
Đặt :
\(A=\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\)
\(\Rightarrow2A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\)
\(\Rightarrow2A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)
\(\Rightarrow2A=\frac{1}{3}-\frac{1}{11}\)
\(\Rightarrow2A=\frac{3}{11}\)
\(\Rightarrow A=\frac{3}{22}\)
1/3 x 5 = 1/3 - 1/5 ; 1/5 x 7 = 1/5 - 1/7 ; làm tương tự với số còn lại
Ta được :
1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11
= 1/3 - 1/11
=
Tính nhanh
2/1x3 + 2/3x5 + 2/5x7 + 2/7x9 +...+ 2/19x21
\(\frac{2}{1\times3}+\frac{2}{3\times5}+...+\frac{2}{19\times21}=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{19}-\frac{1}{21}\)
\(=1-\frac{1}{21}=\frac{20}{21}\)
đúng cái nhé
tính bằng cách thuận tiên nhất
1/1x3 + 1/3x5 + 1/5x7 + 1/7x9 + 1/7x9 +1/9x11
làm rõ
ai nhanh và đúng mình tick cho ha
\(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\)
\(=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\)
\(=\frac{1}{2}\left(1-\frac{1}{11}\right)=\frac{1}{2}.\frac{10}{11}=\frac{5}{11}\)