Question 1160228
sum is (n/2)(2a+d(n-1))
a1=24
a2=24-8, and d=8, n=20
a3=24-(8*2)=8

a20=24-(8*19)=-128
sum is (10)(48+(-8)(19))=10 (-104)=-1040

check
sum 
24/16/8/0/-8/-16/...../-128