Câu 15:
a: \(x^2-4=x^2-2^2=\left(x-2\right)\left(x+2\right)\)
b: \(6x^2+3x=3x\cdot2x+3x\cdot1=3x\left(2x+1\right)\)
c: \(2x^2-4xy+2y-x\)
=2x(x-2y)-(x-2y)
=(x-2y)(2x-1)
d: \(x^2-y^2-6x+9\)
\(=x^2-6x+9-y^2\)
\(=\left(x-3\right)^2-y^2\)
=(x-3-y)(x-3+y)
Câu 17:
\(A=x^2-4x+5\)
\(=x^2-4x+4+1\)
\(=\left(x-2\right)^2+1\ge1\forall x\)
Dấu '=' xảy ra khi x-2=0
=>x=2
Câu 16:
a: 7x-28=0
=>7x=28
=>x=4
b: \(4x^2+20x+25=0\)
=>\(\left(2x\right)^2+2\cdot2x\cdot5+5^2=0\)
=>\(\left(2x+5\right)^2=0\)
=>2x+5=0
=>2x=-5
=>\(x=-\frac52\)
c: \(\left(2x-3\right)\left(4x^2+6x+9\right)=8x\left(x^2-1\right)\)
=>\(8x^3-27=8x^3-8x\)
=>-8x=-27
=>8x=27
=>\(x=\frac{27}{8}\)
