Bonjour
soit x le DVD et y le CD
on a :3x + 25 = 47.50
3x = 47.50 -25
3x = 22.50
x = 7.50
le DVD coûte 7.50 et les CD 5 euros
N°21
a) -2x +7 = 12
-2x = 12-7
-2x = 5
x = -5/2
b
-2+3y = 6
3y = 6+2
3y = 8
y = 8/3
c) 5-2a =12
-2a = 12-5
-2a = 7
a = -7/2
d) -1/2 = 3/2t -5/2
-1/2 +5/2 = 3t/2
4/2 = 3t/2
t = 4/3
N°25
a) 2(x+1) = 3(x-1)
2x+2 = 3x -3
2x-3x = -3-2
-x = -5 d'où x = 5
b) 3(2-3y) = - (y -2)
6--y = -y+2
-y+y = 2-6 donc pas de solution
c) -2(a-1) = 4(2a+1)
-2a+2 = 8a +4
-2a -8a = 4-2
-10a = 2
a = -2/10 = -1/5
