Giải thích các bước giải:
Ta có:
$A=25\cdot \left(-\dfrac12\right)^2+\dfrac15-2\cdot \left(-\dfrac12\right)^2-\dfrac12$
$\to A=25\cdot \left(\dfrac12\right)^2+\dfrac15-2\cdot \left(\dfrac12\right)^2-\dfrac12$
$\to A=25\cdot \dfrac14+\dfrac15-2\cdot \dfrac14-\dfrac12$
$\to A=25\cdot \dfrac14+\dfrac15- \dfrac12-\dfrac12$
$\to A=25\cdot \dfrac14+\dfrac15-\left( \dfrac12+\dfrac12\right)$
$\to A=\dfrac{25}4+\dfrac15-1$
$\to A=\dfrac{125}{20}+\dfrac4{20}-\dfrac{20}{20}$
$\to A=\dfrac{125+4-20}{20}$
$\to A=\dfrac{109}{20}$
