Question

The sum of first four terms of an A.P. is 56 and
the sum of its last four terms is 112. If its first
term is 11, then number of its terms is-
(A) 10
(B) 11
(C) 12
(D) None of these

plz give answer with method
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Answer 1

Answer:

(B)11

Step-by-step explanation:

it is given that

a+(a+d)+(a+2d)+(a+3d)=56

4a+6d=56

sice a=11

6d=12

d=2

Now we are also given

(a+(n-1)d)+(a+(n-2)d)+(a+(n-3)d)+(a+(n-4)d)=122

4a+(4n-10)d=122

as a=11,d=2

then

44+(4n-10)2=122

(4n-10)2=68

4n-10=34

4n=44

n=11