Thay 
bằng biểu thức thích hợp.
a) \(\left( {x - 3y} \right)\left( {x + 3y} \right) = {x^2} - ?\);
b) \(\left( {2x - y} \right)\left( {2x + y} \right) = 4? - {y^2}\);
c) \({x^2} + 8xy + ? = {\left( {? + 4y} \right)^2}\);
d) \(? - 12xy + 9{y^2} = {\left( {2x - ?} \right)^2}\).
Advertisements (Quảng cáo)
Sử dụng ba hằng đẳng thức:
\(\begin{array}{l} + ){A^2} - {B^2} = \left( {A + B} \right)\left( {A - B} \right)\\ + ){\left( {A + B} \right)^2} = {A^2} + 2AB + {B^2}\\ + ){\left( {A - B} \right)^2} = {A^2} - 2AB + {B^2}\end{array}\)
a) \(\left( {x - 3y} \right)\left( {x + 3y} \right) = {x^2} - 9{y^2}\);
b) \(\left( {2x - y} \right)\left( {2x + y} \right) = 4{x^2} - {y^2}\);
c) \({x^2} + 8xy + 16{y^2} = {\left( {x + 4y} \right)^2}\);
d) \(4{x^2} - 12xy + 9{y^2} = {\left( {2x - 3y} \right)^2}\).