А)5х+4=25-2х
5х+2х=25-4
7х=21
х=3
б)3х-35=7х-28
7х-3х=28-35
4х=-7
х=-1,75
Y ' = 3x^2 - 2x - 40
y ' = 0
3x^2 - 2x - 40 = 0
D = 4 + 4*40*3 = 484 = 22^2
x1 = ( 2 + 22)/6 = 24/6 = 4 ∈ [3; 8]
x2 = ( 2 - 22)/6 = - 20/6 = - 10/3 = - 3 ц 1/3 ∉ [3; 8]
y(3) = 27 - 9 - 120 = - 102
y(4) = 64 - 16 - 160 = -112
y(8) = 512 - 64 - 320 = 128
Ответ
y(4) = - 112