Question

if the sum of an 25th term of an ap whose nth term is given by an=7-3n...
plzz solve this...its too urgent...​

Answer 1

Answer:

n = 7 -3n  ------(1)

l1 = 7- 3*1 = 7-3 =4

l2 = 7 - 3*2 = 7 - 6 =1

l3 = 7 - 3*3 = 7 - 9 = -2

first term = a = 4

common difference = d = l2 - l1 = 1-4 = -3

sum of 25 terms = sn = n/2[2a+(n-1)d]

s25 = 25/2 * [ 2*4 + (25-1)(-3)]

 = 25/2 * [8 + 24(-3)]

= 25/2 * [ 8- 72]

= 25/2 * [-64]

= - 25 *32

= -800