Question

find the sum of first 20 term of AP 3 7 11 15 using the formula​

Answer 1

Answer:

a=3

d=4

n=20

Step-by-step explanation:

Sn= n/2 (2a +(n-1)d)

=20/2 (2×3 +(20-1)4)

=10 (82)

= 820

Answer 2

The sum of 20 terms of  the given AP($S_{20}$) = 820

Step-by-step explanation:

The given AP are:

3, 7, 11, 15, ........

Here, first term (a) = 3, common difference(d) = 7 - 3 = 4 and

The number of terms (n) = 20

To find, the sum of 20 terms of  the given AP($S_{20}$) = ?

We know that,

The sum of nth terms of  the AP

$S_{n}=\dfrac{n}{2} [2a+(n-1)d]$

The sum of 20 terms of  the given AP.

$S_{20}$ = $\dfrac{20}{2} [2(3)+(20-1)4]$

= 10(6 + 19 × 4)

= 10(6 + 76)

= 10(82)

= 820

∴ The sum of 20 terms of  the given AP($S_{20}$) = 820