1/(x+1)(x-5)=x²-5x+x-5=x²-4x-5
2/9x²+30x+a=(3x)²+2.3x.5+25=(3x+5)²
=>a=25
3/(x+2)²=x²+4x+4
4/x²+2x+1=(x+1)²=(9+1)²=10²=100
5/A=x²-4x+1=x²-2x.2+4-3=(x-2)²-3
Mà (x-2)²≥0
=>(x-2)²-3≥-3
Dấu = xra ⇔x-2=0=>x=2
Vậy min A=-3 khi x=2
6/x²-2x+1=(x-1)²=(11-1)²=10²=100
CHO CTLHN NHA