bonjour,
A=5x(9x²+4)=
= 45x^3+20x
B=2(6x-4)-(8x²+2x-8)
= 12x-8-8x²-2x+8
= -8x²+10x
C=(7x+5)(4x+8)
= 28x²+56x+20x+40
= 28x²+76x+40
D=(7x-5)(8x+6)
= 56x²+42x-40x-30
= 56x²+2x-30
E=(2x/3-8)(7- 4x/5)
= 14x/3 - 8x²/15 -56+32x/5
= (70x-8x²-840+96x)/15
= (-8x²+166x-840)/15
= (-8x²+166x)/15 - 56