câu 1
a,x² + xy - 7x - 7y b, x²z - 10xz + 25z
=(x²+xy)+(7x-7y) =(x²z-10xz+25z)
=x(x+y)+7(x-y) =z(x²-10x+25)
=x(x+y)-7(x+y) =x(x-5)²
=(x-7)(x+7)
c, x² - 2xy + y² - 9 d,x² - x - 20
=(x-y)²-9 =x²-x-25+5
=(x²-25)-(x+5)
=(x-5)-(x+5)
câu 3
a,9x²-1=0
9x²=0+1
9x²=1
x²=1:9
x²=$\frac{1}{9}$
x=$\frac{1}{3}$
b,x²+5x+4=0
⇒(x-1)(x-4)=0
⇔\(\left[ \begin{array}{l}x=1\\x=4\end{array} \right.\)