Question

The sum of the first 20 terms of an ap whose first term is 5 and common difference is 4 is is what

Answer 1

Step-by-step explanation:

an=a+(n-1) d. an= 5+76=81 sn=n/2(a +an)=10(5+81)=10*86=860

Answer 2

The sum of first 20 terms is 860.

Step-by-step explanation:

Given : An AP whose first term is 5 and common difference is 4.

To find : The sum of the first 20 terms ?

Solution :

The sum of n terms of an A.P is given by,

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

Where, number of terms is n=20

The first term is a=5

The common difference is d=4

Substitute the value in the formula,

$S_{20}=\frac{20}{2}[2(5)+(20-1)4]$

$S_{20}=10[10+(19)4]$

$S_{20}=10[10+76]$

$S_{20}=10[86]$

$S_{20}=860$

Therefore, the sum of first 20 terms is 860.

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