Tu dois utiliser la distributivité : (a+b)(c+d) = ac +ad + bc +bd.
[tex]a = ( 2x + \frac{3}{7} )( \frac{2}{3} - 4x) \\ a = \frac{4x}{3} - 8 {x}^{2} + \frac{6}{21} - \frac{12x}{7} \\ a = - 8 {x}^{2} - \frac{12x \times 3}{7 + \times 3} + \frac{4x \times 7}{3 \times 7} + \frac{6}{21} \\ a = - 8 {x}^{2} - \frac{36x}{21} + \frac{28x}{21} + \frac{6}{21} \\ a = - 8 {x}^{2} - \frac{8x }{21} + \frac{2}{7} [/tex]
[tex]b = 3x - (5x - 1)(2 - x) \\ b = 3x - (10x - 5 {x}^{2} - 2 + x) \\ b = 3x - 10x + 5 {x}^{2} + 2 - x \\ b = 5 {x}^{2} - 6x + 2 \\ b = 5 {x}^{2} - 2( - 3x + 1)[/tex]
