Question

Given the arithmetic series 30+25+20+15+10... Find the sum of the first n terms when n=42

Answer 1

$S(n) = \frac{n}{2}(2a(1)+(n-1)d)$
What is a(1)? 30.
What is d? Seems to be -5.
What is n? 42
So...
$S(42) = 21((2*30)+(41*-5))$
$S(42) = 21(60-205)$
$S(42)= -3045$