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a)x(1/2-2/3)=7/12
x.(-1/6)=7/12
x=7/12 / (-1/6)
x=-7/2
b)x=13/15 / 5,5
x=26/165
c)3x/7 + 1=-1/28.(-4)
3x/7 + 1=1/7
3x/7=1/7-1=-6/7
3x=-6/7.7=-6
x=-6:3=-2
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7/1.3 + 7/3.5 + 7/5.7 + ... + 7/99.101
= 7.(1/1.3 + 1/3.5 + 1/5.7 + ... + 1/99.101)
= 7/2 . 2 . (1/1.3 + 1/3.5 + 1/5.7 + ... + 1/99.101)
= 7/2 . (2/1.3 + 2/3.5 + 2/5.7 + ... + 2/99.101)
= 7/2 . (1 - 1/3 + 1/3 - 1/5 + ... + 1/99 - 1/101)
= 7/2 . (1 - 1/101)
= 7/2 . 100/101
= 350/101
\(\frac{7}{1.3}+\frac{7}{3.5}+...+\frac{7}{99.101}\)
\(=7\left(\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{99.101}\right)\)
\(=\)\(\frac{7}{2}.2.\left(\frac{1}{1.3}+\frac{1}{3.5}+...+\frac{1}{99.101}\right)\)
\(=\)\(\frac{7}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{99.101}\right)\)
x*(5/10-2/3)=7/12
x*-1/6=7/12
x=7/12:-1/6
x=-7/2
làm như vậy đó