Question

The sum of first 20 terms of an ap in which third term is 3 and 7th term is 2 more than thrice of its 3rd term is

Answer 1

Step-by-step explanation:

a3=3

a+2d = 3

a=3-2d

now,

a+6d =a3+2

3-2d+6d=3-2d+2d+2

4d=2

d=1/2

now putting the value of d in a+2d

a=3

now, sn=n/2= (2a+(n-1)d)

20/2×(2×3+19×1/2)

10×12.5 =125

hence sum of of 20 terms of an ap is 125

Answer 2

Answer:

3rd term is

a+2d=3 ---1

7th term is

a+6d --2

according to the question

(a+6d) =3(a+2d) +2

2a=-2

a=-1

d=2

sum of the 20 terms is

20/2 ×(-2+19×d)

=10×36

=360

ok

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