Question

In an ap the sum of n terms is 1by2(5n2+3ng find 40th term

Answer 1

Hi ,

Sum of n terms = Sn = ( 5n² + 3n ) /2

first term = a = S1 = ( 5 + 3 ) / 2 = 8

S2 = [ 5 × 2² + 3 × 2 ] /2 = 13

second term = a2 = S2 - S1

a2 = 13 - 8 = 5

common difference = a2 - a1

d = 5 - 8 = -3

40th term = a40 = a + 39d

= 8 + 39 ( - 3 )

= 8 - 117

= -109

Therefore ,

required 40th term = a40 = -109

I hope this helps you.