Thực hiện phép tính ( hợp lí nếu có thể): a, 25.(-1/2)mũ 2 + 1/5-2.(-1/2)mũ 2 – 1/2

Giải thích các bước giải:

Ta có:
$A=25\cdot \left(-\dfrac12\right)^2+\dfrac15-2\cdot \left(-\dfrac12\right)^2-\dfrac12$ 

$\to A=25\cdot \left(\dfrac12\right)^2+\dfrac15-2\cdot \left(\dfrac12\right)^2-\dfrac12$ 

$\to A=25\cdot \dfrac14+\dfrac15-2\cdot \dfrac14-\dfrac12$ 

$\to A=25\cdot \dfrac14+\dfrac15- \dfrac12-\dfrac12$ 

$\to A=25\cdot \dfrac14+\dfrac15-\left( \dfrac12+\dfrac12\right)$ 

$\to A=\dfrac{25}4+\dfrac15-1$ 

$\to A=\dfrac{125}{20}+\dfrac4{20}-\dfrac{20}{20}$ 

$\to A=\dfrac{125+4-20}{20}$ 

$\to A=\dfrac{109}{20}$