bonjour,
a) V1(x) = 1/3(15*10*x) = 150x/3 =50x
b) V2(x) = 10*15*(12-x) = 150(12-x) = 1800-150x
c) V3 = 50x+180-150x = -100x+1800
3) par le calcul, tu retrouves sur ton graph
V1 = 50*4 = 200
V2 = 1800-150*4 = 1200
V3 = -100*4+1800 = 1400
b) 50x = 1800-150x
50x+150x = 1800
200x = 1800
x = 9