Đáp án:
\(\begin{array}{l} a,\ -(5+4y)(5-4y)=-25+16y^2\\ b,\ (2x-3)(2x+3)=4x^2-9\\ c,\ \bigg(\dfrac12x-2y\bigg)\bigg(\dfrac12x+2y\bigg)=\dfrac14x^2-4y^2\end{array}\)
Giải thích các bước giải:
\(\begin{array}{l} a,\ -(5+4y)(5-4y)\\ =-\left [5^2-(4y)^2\right ]\\ =-(25-16y^2)\\ =-25+16y^2\\ b,\ (2x-3)(2x+3)\\ =(2x)^2-3^2\\ =4x^2-9\\ c,\ \bigg(\dfrac12x-2y\bigg)\bigg(\dfrac12x+2y\bigg)\\ =\bigg(\dfrac12x\bigg)^2-(2y)^2\\ =\dfrac14x^2-4y^2\end{array}\)
chúc bạn học tốt!