Question

In an AP if the 18th burm us - 13 and sum of it
first four term us 24, find the sum of its
first den terme​

Answer 1

Step-by-step explanation:

ANSWER

Given that :-

12th term of the A.P is −13

sum of firts four terms is 24

Let, the 1st term of the A.P is a.

and common difference of the A.P is d.

We know, the formula for nth of an A.P. is ,

tn=a+(n−1)d

and, the formula for the sum of n−terms of an A.P. is,

Sn=2n[2a+(n−1)d]

By the question,

t12⇒a+(12−1)d=−13

⇒a+11d=−13

⇒a=−13−11d------(i)

S4→24[2a+(4−1)d]=24

⇒2a+3d=12

⇒a=212−3d------(ii)

comparing (i) and (ii) we get,

−13−11d=212−3d 

    

⇒−22d+3d=26+12   

⇒−19d=38          ⇒d=−2

Now, from (i) we get, 

a=

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