= ex1
A = (1/8 +7/12)x(6/5 ÷4/15)
on commence par la division 6/5 ÷ 4/15 = 6 x 15/4 x 5 = 2 x 3 x 3 x 5/2 x 2 x 5
= 9/2
1/8 + 7/12 = 3/24 + 14/24 = 17/24
A = (17/24) x 9/2 = 17 x 9/24 x 2 = 153/48
B = 1/8 + 7/12 ÷ 7/6 + 7/12
7/12 ÷ 7/6 = 7 x 6/7 x 12 = 7 x 6/7 x 2 x 6 = 1/2
B = 1/8 + 1/2 + 7/12
on commence les opérations par la gauche
1/8 + 1/2 = 1/8 + 4/8 = 5/8
B = 5/8 + 7/12 = 5 x 3/24 + 2 x 7/24 = 15/24 + 7/24 = 22/24 = 11/12
EXN°2
A = (5x + 2)² - (3x - 1)(5x +2)
1. Développer et réduire A
(5x + 2)² = 25x² + 20x + 25
identité remarquable (a + b)² = a² + 2ab + b²
a = 5x ⇒a²= 25x²
b = 2 ⇒b² = 4
2ab = 2(5x)(2) = 20x
(3x - 1)(5x +2) = 15x² + 6x - 5x - 2 = 15x² + x - 2
A = 25x² + 20x + 25 - (15x² + x - 2)
= 25x² + 20x + 25 - 15x² - x + 2
= 10x² + 19x + 27
3) calculer A pour x = 0 et x = 3/2
pour x = 0 : A = 27
pour x = 3/2 A =10(3/2)² + 19(3/2) + 27
= 10 x 9/4 + 19 x 3/2 + 27
= 2 x 5 x 9/4 + 19 x 3/2 + 27 = 45/2 + 57/2 + 27 = 51 + 27 = 78