Question

What is the sum of the first 20 terms of the series 10 +7+4+1+....

Answer 1

Here,

a = first term = 10

d  = common difference = (-3)

n  = no of terms = 20

Sum of terms = $\frac{n}{2}$ (2a + (n-1) d)

                       = $\frac{20}{2}$ (2(10) + (20-1)(-3))

                       = 10(20 -19(3))

                       = 10(20 - 57)

                       = 10(-37)

                       = -370