Giải bài 4 trang 26

Đề bài

Tính một cách hợp lí:

a) $\frac{4}{15}-\left(2,9-\frac{11}{15}\right)$;

b) $(-36,75)+\left(\frac{37}{10}-63,25\right)-(-6,3)$;

c) $6,5+\left(-\frac{10}{17}\right)-\left(-\frac{7}{2}\right)-\frac{7}{17}$;

d) $(-39,1)\cdot \frac{13}{25}-60,9\cdot \frac{13}{25}$.

Lời giải chi tiết

a)

 $\begin{array}{c}\frac{4}{15}-\left(2,9-\frac{11}{15}\right)\\ =\frac{4}{15}-2,9+\frac{11}{15}\\ =\left(\frac{4}{15}+\frac{11}{15}\right)-2,9\\ =\frac{15}{15}-2,9\\ =1-2,9=-1,9\end{array}$

b)

$\begin{array}{c}(-36,75)+\left(\frac{37}{10}-63,25\right)-(-6,3)\\ =(-36,75)+3,7-63,25+6,3\\ =\left(-36,75-63,25\right)+\left(3,7+6,3\right)\\ =-100+10=-90\end{array}$

c)

$\begin{array}{c}6,5+\left(-\frac{10}{17}\right)-\left(-\frac{7}{2}\right)-\frac{7}{17}\\ =\frac{65}{10}-\frac{10}{17}+\frac{7}{2}-\frac{7}{17}\\ =\left(\frac{65}{10}+\frac{7}{2}\right)-\left(\frac{10}{17}+\frac{7}{17}\right)\\ =\left(\frac{65}{10}+\frac{35}{10}\right)-\frac{17}{17}\\ =\frac{100}{10}-1\\ =10-1=9\end{array}$

d)

$\begin{array}{c}(-39,1)\cdot \frac{13}{25}-60,9\cdot \frac{13}{25}\\ =\frac{13}{25}.\left(-39,1-60,9\right)\\ =\frac{13}{25}.\left(-100\right)\\ =-52\end{array}$.