Question

What is the sum of the first 19 terms of the series: 63 + 55 + 47 + 39 + 31 + …?

Answer 1

-171

Answer 2

i think they want us to explain a little
this is an arithmetic series, with common difference of d = -8 , first term is a= 63
n = 19
Sn = 0.5n(2a + (n-1)d)

substituting in our values we get

S_19 = 0.5(19)(2(63) + (19-1)(-8)) = -171