Đáp án+Giải thích các bước giải:
11) (x+4).(x-2)-(x-3)²
= (x²-2x+4x-8)-(x²-2.x.3+3²)
= x²-2x-8-x²-6x+9
= (x²-x²)-(2x+6x)+(9-8)
= -8x+1
12) (x-2)²+(x-1).(x+5)
= (x²-2.x.2+2²)+(x²+4x-5)
= x²-4x+4+x²+4x-5
=(x²+x²)+[(-4x)+4x]+(4-5)
= 2x²-1
13) (x+2).(x-3)+x.(x+1)
=(x²-3x+2x-6)+x²+x
=x²-x-6+x²+x
=(x²+x²)+[(-x)+x]-6
=2x²-6
14) (x+3).(x-3)-x.(23+x)
= (x²-3x+3x-9)-23x-x²
= x²-9-23x-x²
=(x²-x²)-23x-9
=-23x-9
15) 2x.(1-x)-(2x-1).(x+1)
= 2x-2x²- 2x²-2x-x-1
= (2x-2x)-(2x²+2x²)+x+1
= -4x²+x+1
