Question

Find the sum of first 16 terms of an Arithmetic Progression whose
4th and 9th terms are
-
- .​

Answer 1

Answer:

4th term = a + 3d = 15

9th term = a + 8d = -30

- - +

------------------------

-5d = 45

d = -9

so, a = 15-3(-9) = 15+27 = 42

so, A16 = a+(n-1)d = 42 + (16-1)(-9)

= 42-135

= -93

so, S16 = 8(42-93)

= 8(-51)

= -408 ... ans