câu 1:
a) (x-2y)²=x²-2x.2y+(2y)²
b) (x+5y)²=x²+2x.5y+(5y)²
c) (x-5)³=x³-3.x².5+3.x.5²-5³
d) (x-2)³=x³-3.x².2+3.x.2²-2³
câu 2:
a) (2x-1)(2x+1)-5x²=(2x)²-1-5x²
=4x²-5x²-1
=(-x²)-1
b) (2x+1)²-5x(x+5)=4x²+2.2x+1-5x²-25x
=4x²+4x+1-5x²-25x
=(-x)²-21x+1
câu 3:
a) (x-3)(x+3)-x(x-5)=1
⇒ x²-9-x²+5x=1
⇒ 5x-9=1
⇒ 5x=10
⇒x=2
b) 5x(x-3)-5x²+9=0
⇒ 5x²-15x-5x²+9=0
⇒-15x=-9
⇒15x=9
⇒x= 3 phần 5
