Question

Find the sum of first 15 terms of an ap whose nth term is 6-n

Answer 1

Answer:

Step-by-step explanation:

   Given nth term of AP an = 3 - 2n

Put n = 1, we get

a1 = 3 - 2 = 1

Put n = 2, we get a2 = 3 - 4 = - 1

Hence d = -1 - 1 = -2

a15 = 3- 30 =  - 27

Sum of 15 terms = (15/2)[1 + (-27)]

= (15/2) × - 26 = -195

Answer 2

given

nth term of a. p is 6-n

if n=1

then

6-n=6-1=5

if n=2

then

6-n=6-2=4

if n=3

then

6-n=6-3=3

Therefore we get

a1=5 a2=4

d=4-5=-1

S15=n/2(2a+(n-1)d)

15/2(2.5+(15-1)-1)

15/2(10-14)

15/2(-4)

15 . -2

-30

:.S15 is -30.

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