Question

Write the sum using summation notation, assuming the suggested pattern continues. -9 - 3 + 3 + 9 + ... + 81

Answer 1

-9 -3 +3 +9 +15 +....+81=15+21+...+81=(23*66)/2=759

Answer 2

Honestly I believe this is correct:

∑ 15 at the top, n =0 (-9+6n)