Question

-2,-7,-12 ,.... is an arithmetic sequence Find the sum of its first 20 terms ?​

Answer 1

Given sequence:

-2,-7,-12 ,..

Unknown;

Find the sum of the first 20 terms = ?

Solution:

The sequence is -2,-7,-12 ,..

To find he sum of n-terms of a sequence, we use the expression below;

Sₙ =
`(n)/(2) (2a + (n-1)d)`

where n is the number of terms = 20

a is the first term = -2

d is the common difference= -7 - (-)2 = -5

Now input the parameters and solve;

Sₙ =
`(20)/(2) (2(-2) + (20 - 1) x -5)`

Sₙ = 10(-4 + -5(19)) = 10(-99) = -990

The sum of the first 20 terms of the sequence is -990