a7=-100, a9=-78, a15=?. s15=?
a7+2d=a9, 2d=a9-a7, 2d=-78-(-100)=-78+100=22
d=22:2=11
a15=a9+6d, a15=-78+66=-12
a15=-12
sn=n(a1+an)/2
a1=a7-6d, a1=-100-66=-166
s15=15(-166-12)/2=15(-178)/2=15(-89)=--1335
s15=-1335
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a7=-100, a9=-78, a15=?. s15=?
a7+2d=a9, 2d=a9-a7, 2d=-78-(-100)=-78+100=22
d=22:2=11
a15=a9+6d, a15=-78+66=-12
a15=-12
sn=n(a1+an)/2
a1=a7-6d, a1=-100-66=-166
s15=15(-166-12)/2=15(-178)/2=15(-89)=--1335
s15=-1335