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= 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... 1/9.10
= 1 - 1/10
= 9/10
Bạn xem lại chỗ 1/10
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a) 6 + 2x = 12 +(-5)
2x = 7 -6
x = 1/2
b) -90 - |x| -10 + 100 = 0
- |x| =0
x =0
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1/2 + 5/6 + 11/12 + ... + 89/90 + 109/110 + 10/11
= (1 - 1/2) + (1 - 1/6) + (1 - 1/12) + ... + (1 - 1/90) + (1 - 1/110) + 10/11
= (1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1) - (1/1*2 + 1/2*3 + 1/3*4 + ... + 1/10*11) + 10/11
= 9 - (1 - 1/11) + 10/11
= 9 - 10/11 + 10/11 = 9
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\(\frac{5}{6}+\frac{11}{12}+\frac{19}{20}+...+\frac{89}{90}\)
\(=1-\frac{1}{6}+1-\frac{1}{12}+1-\frac{1}{20}+...+1-\frac{1}{90}\)
\(=8-\left(\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)
\(=8-\left(\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)
\(=8-\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{9}-\frac{1}{10}\right)\)
\(=8-\left(\frac{1}{2}-\frac{1}{10}\right)\)
\(=\frac{38}{5}\)
nó trã lời đại mà cũng li ke ngu