=> [( x+y )+ (x-y)]2
=> ( x+y+x-y )2
=> (2x)2
=> 4x2
(x+y)2+2(x+y)(x-y)+(x-y)2
=(x+y+x-y)2
=(2x)2
=4x2
áp dụng công thức :\(A^2+2AB+B^2\)
Ta có :\(\left(x+y\right)^2+2\left(x+y\right)\left(x-y\right)+\left(x-y\right)^2\)
=\(\left(x+y+x-y\right)^2=x^2\)