Répondre :
A=2[(x+1)(x+2)-(x-1)(x+3)]
=2[(x²+2x+x+2)-(x²+3x-x-3)]
=2(x²+3x+2-x²-2x+3)
=2x+10
B=(x+y)(3x+2xy-1)
=3x²+2x²y-x+3xy+6xy²-y
=3x²+2x²y+6xy²+3xy-x-y
C=(x²+3)(x²-9)
=x^4-9x²+3x²-27
=x^4-6x²-27
=2[(x²+2x+x+2)-(x²+3x-x-3)]
=2(x²+3x+2-x²-2x+3)
=2x+10
B=(x+y)(3x+2xy-1)
=3x²+2x²y-x+3xy+6xy²-y
=3x²+2x²y+6xy²+3xy-x-y
C=(x²+3)(x²-9)
=x^4-9x²+3x²-27
=x^4-6x²-27