Question

the n th term of an ap is given by (- 4 n
+ 15) find the sum of the first 20 term of this AP​

Answer 1

Answer:

Step-by-step explanation:

A/Q

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

Thanks!! :)