Chọn dấu “=”, “≠” thích hợp cho ô hỏi chấm (3,9+1/2.2,6):13 hỏi chấm 3,9 + 1/2. 2,6 : 13

* Hướng dẫn giải

$$\begin{gathered} a) Ta có \left(3,9+\frac{1}{2}.2,6\right):13 \\ =3,9:13+\frac{1}{2}.2,6:13=0,3+\frac{1}{2}.2,6:13 \\ Vì ,3 \ne 3,9 nên \left(3,9+\frac{1}{2}.2,6\right):13\ne 3,9+\frac{1}{2}.2,6:13 \\ Vậy \left(3,9+\frac{1}{2}.2,6\right):13\boxed{\ne }3,9+\frac{1}{2}.2,6:13 \end{gathered}$$

$$\begin{gathered} b) \frac{5}{4}-{\left(\frac{1}{4}\right)}^2=\frac{5}{4}-\frac{1^2}{4^2}=\frac{5}{4}-\frac{1}{16}=\frac{20}{16}-\frac{1}{16}=\frac{19}{16} \\ {\left(\frac{5}{4}-\frac{1}{4}\right)}^2=1^2=1 \\ Vì \frac{19}{16}\ne 1 nên \frac{5}{4}-{\left(\frac{1}{4}\right)}^2\ne {\left(\frac{5}{4}-\frac{1}{4}\right)}^2 \\ \text{Vậy }\frac{5}{4}-{\left(\frac{1}{4}\right)}^2\boxed{\ne }{\left(\frac{5}{4}-\frac{1}{4}\right)}^2 \end{gathered}$$

$$\begin{gathered} c) Ta có \frac{28}{9}.0,7+\frac{28}{9}.0,5=\frac{28}{9}.(0,7+0,5) \\ \frac{28}{9}.0,7+\frac{28}{9}.0,5\boxed{=}\frac{28}{9}.(0,7+0,5) \end{gathered}$$

$$\begin{gathered} d) Ta có \frac{36}{13}:4+\frac{36}{13}:9=\frac{36}{13}.\frac{1}{4}+\frac{36}{13}.\frac{1}{9} \\ =\frac{36}{13}.\left(\frac{1}{4}+\frac{1}{9}\right)=\frac{36}{13}.\left(\frac{9}{36}+\frac{4}{36}\right)=\frac{36}{13}.\frac{13}{36} \\ \frac{36}{13}:(4+9)=\frac{36}{13}:13=\frac{36}{13}.\frac{1}{13}=\frac{36}{13}.\frac{3}{36} \\ \text{Vì }\frac{13}{36}\ne \frac{3}{36} nên \frac{36}{13}.\frac{13}{36}\ne \frac{36}{13}.\frac{3}{36} \\ \text{Vậy }\frac{36}{13}.\frac{13}{36}\ne \frac{36}{13}.\frac{3}{36} \end{gathered}$$