Question

The 10th term of an AP is -27 and the 5th term is -12, what is the 18th term?

Also find the sum of its 25 terms.​

Answer 1

a+9d=-27......(1)

a+4d=-12.....(2)

make a the subject of equation 2 and put into equation 1

-12+4d+9d=-27

-12+13d=-27

d=-15\13

put d into equation 1

a+9(-15/13)=-27

a=16.6

a+17d=x

17+16.6(-15/13)

x=-2 is the 18th term

25÷2(16.6+15.6(-15/13)

=-208.4

Answer 2

Answer:10th term: -27

5th term: - 12

d= -27+12/5

= -3

a+(n-1)d=-12

a+(5–1)d=-12

a+4d= -12

a=-12–4d

a=-12-(4×-3)

a=-12+12=0

18 term=a + (18–1)d

=0+(17×-3)

51