bonsoir
a1= 4x(7x-8)=28x²-32x
a2=5y(9-7y)=45y-35y²
a3=4n(7-5n)+7n²+8n-9=28n-20n²+7n²+8n-9=-13n²+36n-9
a4=-3x(7x-9)+7x(4-9x)=-21x²+27x+28x-63x²=-84x²+55x
a5=4x²-7x+9+(7x²-9x+8)-(-3x²-8x-12)=4x²-7x+9+7x²-9x+8+3x²+8x+12=14x²-8x+29
a6=-4x²-7x+9-(9x²-7x+25)=-4x²-7x+9-9x²+7x-25=-13x²-16
a7=7x(9x-5)-3x(-4x+9)-4x²-5x+9=63x²-35x+12x²-27x-4x²-5x+9=71x²-67x+9
a8=-(3x-5)+4(3x²-7x+9)-(7x²-12x-15)=-3x+5+12x²-28x+36-7x²+12x+15=5x²-19x+56