Giải bài 3 trang 26

Đề bài

Chọn dấu " "=", " $\ne$ " thích hợp cho dấu “?” :

a) $\frac{28}{9}\cdot 0,7+\frac{28}{9}\cdot 0,5$ ? $\frac{28}{9}\cdot (0,7+0,5)$;

b) $\frac{36}{13}:4+\frac{36}{13}:9$ ? $\frac{36}{13}:(4+9)$.

Lời giải chi tiết

a)

 $\frac{28}{9}\cdot 0,7+\frac{28}{9}\cdot 0,5=\frac{28}{9}.\left(0,7+0,5\right)$

b)

$\begin{array}{c}\frac{36}{13}:4+\frac{36}{13}:9\\ =\frac{36}{13}.\frac{1}{4}+\frac{36}{13}.\frac{1}{9}\\ =\frac{36}{13}.\left(\frac{1}{4}+\frac{1}{9}\right)\\ =\frac{36}{13}.\frac{13}{36}=1\end{array}$

 $\begin{array}{c}\frac{36}{13}:(4+9)\\ =\frac{36}{13}:13\\ =\frac{36}{13}.\frac{1}{13}\\ =\frac{36}{169}\end{array}$

Suy ra $\frac{36}{13}:4+\frac{36}{13}:9$ $\ne$ $\frac{36}{13}:(4+9)$.