Câu 1:$(4x^2 + y^2)(2x+y)(2x-y)$
$= (4x^2 +y^2 )(4x^2 -y^2)$
$= 16x^4 - y^4$
Câu 2:
$(7x+1)^2 - (x+7) ^2 = 48(x^2 -1) (1)$
$<=> 49x^2 +14x+1- (x^2+14x+49) = 48x^2 -48$
$<=> 49x^2 +14x+1- x^2-14x-49= 48x^2 -48$
$<=> 48x^2 -48=48x^2 -48 (đúng) $
Vậy (1) đúng
Câu 3:
$P=(x+3)^2+(x-3).(x-3)-2(x+2)(x-4)$ $với x =-1/2$
$<=> (x+3)^2+(x-3)^2-2(x+2)(x-4)$
$<=> (-1/2+3)^2 + (-1/2-3)^2 - 2.(-1/2+2)(-1/2-4)$
$<=>32 $