Question

b. The nth term of an A.P. is given by (- 4n +15) Find the sum
of first 20 terms of this A.P.
o​

Answer 1

a1 = -4 × 1 + 15 = 11

a2= -4 × 2 + 15 = 7

so , d = a2 - a1

= 7 - 11 = -4

Now

S 20 = 20/2 [2 × 11 + (20-1) -4]

= 10 [ 22 - 76 ]

= 10 ×[-54]

= -540. Hope it helps you