a(1) = 5
a(2) = 11
a(7) -?
S(7) -?
d = a(2) - a(1)
d = 11 - 5 = 6
a(n) = a(1) +d(n-1)
a(7) = a(1) +d * 6
a(7) = 5 + 6*6 = 5+36 = 41
S(n) = [a(1) + a(n)] / 2 * n
S(7) = [a(1) + a(7)] / 2 * 7
S(7) = (5 + 41) / 2 * 7 = 46 / 2 * 7 = 23 * 7 = 161
4х-12=х+15
4х-х=12+15
3х=27
х=9
P=a+b+c+d
6x+8x+10x+5x=58
29x=58
x=58:29
x=2
a=6*2=12
b=8*2=16
c=10*2=20
d=5*2=10