Question

in an ap an = 2-3n find the sum of the first 12 terms​

Answer 1

Lets find the terms of the ap:

an is gn as 2-3n

Substitute,m 1,2,3 and so on for n

So, we will get,

a1=-1, a2=-4, a3=-7 and a4=-10 and so on

Common difference is -3

So, an=a+(n-1)d

=> -1+(12-1)-3 ( we take n=12 because we consider the last term as 12th term as per the qn)

=> -34=an

S12= n/2(a+an)

=> 12/2(-1-34)

=>6*-35

=>-210..... is ur ans:)

Answer 2

Heya!

An = 2 - 3n

put n = 1 to get ist term

A1 = 2 - 3×1 = -1

put n = 2 to get 2nd term

A2 = 2 - 3×2 = -4

Put n = 3 to get 3rd term

A3 = 2 - 3×3 = -7

.

.

.

put n = 12 to get 12th term

A12 = 2 - 3×12 = -34

So, Series becomes

-1 , -4 , -7 , ... , -34

It's sum is given by

S12 = 6 { -1 -34 }

S12 = -210

So, sum of ist 12 term is -210