i dont think that is the right answer the pride.. i see what you did though. you plugged 11 into n. you used the second equation. thanks for helping. the answer is will not help me if i don't understand this. can someone confirm his answer with a little explanation?
the correct problem is:
ok so now i plug that into the three equations.
(-2)(15)-(15(15+1))/2+15(15+1)^2)/2 the i get:
-30-240/2+1920 which equals =1770 but this does not match the answer in my book. i must have done something wrong? can you confirm please?