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\(A=-\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{25.27}\right)-\frac{1}{27}\)
\(=-\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{25}-\frac{1}{27}\right)-\frac{1}{27}\)
\(=-\left(1-\frac{1}{27}\right)-\frac{1}{27}\)
\(=-1+\frac{1}{27}-\frac{1}{27}\)
\(=-1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
\(A=-\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{25.27}\right)-\frac{1}{27}\)
\(=-\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{25}-\frac{1}{27}\right)-\frac{1}{27}\)
\(=-\left(1-\frac{1}{27}\right)-\frac{1}{27}\)
\(=-1+\frac{1}{27}-\frac{1}{27}\)
\(=-1\)
![](https://rs.olm.vn/images/avt/0.png?1311)
a, \(x+y+z-x-y+z=2z\)
Thay z=-10 Thì \(2z=-20\)
b, \(a-b-c+a+b=2a-c=2\cdot\left(-12\right)-\left(-27\right)=3\)
![](https://rs.olm.vn/images/avt/0.png?1311)
Lời giải:
Nếu $x\geq 1$ thì:
$|x-1|-|x+2|=(x-1)-(x+2)=-3$
Nếu $-2\leq x< 1$ thì:
$|x-1|-|x+2|=1-x-(x+2)=-1-2x$
Nếu $x<-2$ thì:
$|x-1|-|x+2|=1-x+x+2=3$
Chọn B