Question

if the 12th term of an AP is -13 and the sum of the first four terms is 24 , find the sum of the first 15 term

Answer 1

Answer:

a+11d= -13 (1)

s4 = 24

4/2(a+(4-1)d)= 24

2(a+3d) = 24

a+3d= 12 (2)

a + 11d = -13

a + 3d = 12

- - -

8d = -25

d = -25/8