A=(1-7x)au carré - (1-7x)(4x+5) 1.developper et reduire A 2. Factoriser A

(1-7x)² -(1-7x)(4x+5)
1-14x+49x² -(4x+5-28x²-35x)
1 -14x+49x² +28x²+31x-5
77x² +17x -4

factoriser

(1-7x)² -(1-7x)(4x+5)
(1-7x)(1-7x-4x-5)
(1-7x)(-11x-4)

1)
A = (1-7x)²-(1-7x)(4x+5)
A = (1-14x+49x²)-(4x+5-28x²-35x)
A = 1-14x+49x²+31x-5+28x²
A = 77x²+17x-4

2) on met 1-7x en facteur
A = (1-7x)(1-7x-4x-5)
A = (1-7x)(-11x-4)