Question

The nth term of an AP is (-4n+15) find the sum of first 20 terms of the AP.

Answer 1

Answer:

-540

Step-by-step explanation:

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

I hope it helps you

Answer 2

Answer:

nth term of an A.P(an) = -4n+15

∴ a1 = -4(1)+15 = 11

a2 = -4(2)+15 = 7

d = a2- a1 = 7-11 = -4

And, Sum of first n terms of AP is given by-

Sn = (n/2)[2a+(n-1)d]

= (n/2)[2×11 + (n-1)(-4)]

= (n/2)[22-4n+4]

= (n/2)(26-4n)

= n(13-2n)

∴ S20 = 20(13-2×20) = 20(13-40) = 20(-27) = -540

Thus, the Sum of first 20 terms of AP = -540