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 = [tex]\frac{n}{2} (2a + (n-1)d)[/tex]
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 = [tex]\frac{20}{2} (2(-2) + (20 - 1) x -5)[/tex]
Sₙ = 10(-4 + -5(19)) = 10(-99) = -990
The sum of the first 20 terms of the sequence is -990
