Question

Find the sum of 20 + 14 + 8 + ... + (-70)

Answer 1

-400

Explanation:

20 + 14 + 8 + ... + (-70) =

= (-6·1+26)+(-6·2+26)+(-6·3+26)+...+(-6·16+26) =

= -6·(1+2+3+...+16)+26·16 =

= -6·16·17/2+26·16 =

= -6·8·17+26·16 =

= -816+416 =

= -400