Đáp án + Giải thích các bước giải:
$a)\; \left(3x-\dfrac23\right)^2=(3x)^2-2.3x.\dfrac23+\left(\dfrac23\right)^2=9x^2-4x+\dfrac{4}{9}$
$b)\; 16x^2-81x^2=(4x)^2-(9x)^2=(4x-9x)(4x+9x)=-5x.13x=-65x^2$
$c)\; \dfrac1{25}-36y^2=\left(\dfrac15\right)^2-(6y)^2=\left(\dfrac{1}{5}-6y\right)\left(\dfrac{1}{5}+6y\right)$