Question 1086929
a1=87 common difference is -15
a20=a1+(19)(-15)=87-285=-198
sum is n(a1+an)/2
=10(-111)
=-1110
87
72
57
42
27
12--positive add to 297
-3
-18
-33
-48
-63
-78
-93
-108
-123
-138
-153
-168
-183
-198--negative add to -1407
-1110 is sum.