1)
b1 = 27; b2 = 24
bn = b1+(n-1)d
bn = bn-1+d
d = b2-b1 = -3
b21 = 27-3*20 = -47
2)
c2 = -9; c3 = -5
d = c3-c2 = 4
c1 = c2-d = -13
Sn = n(2c1+d(n-1))/2
S8 = (-26+4(8-1))*8/2 = 8
пусть х - скорость тогда
S/x=2.5
S/(x+1)=2
S=2.5x
S=2(x+1)
S=S
2.5x=2x+2
0.5x=2
x=4 км/час
S=2.5*4=10км