Ответ:

1) n = 26;

2)d = 44/17 = 2 10/17,

c(27) = 91 5/17,

s(10) = 356 8/17,

s(5-10) = 244 16/17

Объяснение:

1) x(n)=x(1)+d*(n-1)

39 = 6,5+1,3*(n-1), n—?

39—6,5 = 1,3(n-1)

32,5=1,3(n-1)

n-1=32,5/1,3

n-1=25

n=25+1

n=26

2) c(18)=c(1)+(18-1)*d

68=24+17d

68-24 = 17d

44 = 17d

d = 44/17 = 2 10/17

с(27)=с(1)+(27-1)*d=c(1)+26*d=

=24 + 26*44/17 = 24 + 1144/17=

=24 + 67 + 5/17 = 91 5/17

s(10)=(c(1)+c(10))*(10-1+1)/2=

=(c(1)+c(1)+(10-1)*d)*5=

=10c(1) + 45*d=

=10*24 + 45*44/17=

=240 + 1980/17 = 240 + 116 + 8/17=

=356 8/17

s(5-10) = (c(5)+c(10))*(10-5+1)/2=

=(c(5)+c(10))*3=

=(c(1)+4*d+c(1)+9*d)*3=

=6c(1)+39d=

=6*24+39*44/17=

=144 + 1716/17 = 144 + 100 + 16/17 =

=244 + 16/17 = 244 16/17