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a7+a3=20
2*a3=12/:2
a3=6
a7+6=20
a7=14
6=a1+2r
14=a1+6r
a1=6-2r
14=(6-2r)+6r
a1=6-2*2
a1=2
a15= 2+14*2=30
S15=(2+30/2)*15=240
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