Question

find the sum of the first 20 terms of the series 4+7+10......

Answer 1

This series is A.P.

so use summation formula..

Sn=
$\frac{n}{2} (2a(n - 1)d)$
here n=20
d=3
by using this in formula-

$= \frac{20}{2} (2 \times 4 + (20 - 1)3)) \\ 10(8 + 57) \\ 650$

$\red{\boxed{\huge{Answer\:is\:650}}}$

Answer 2

The sum is

91

Explanation:

It is the sum of an arithmetic progression (series) with first term

a

1

=

4

and ratio

r

=

3

The sum is

S

7

=

7

2

⋅

(

a

1

+

a

7

)

=

7

2

⋅

(

4

+

22

)

=

7

⋅

13

=

91