a₁=-4·1+13=9
a₂=-4·2+13=5
d=a₂-a₁=5-9=-4
a₁₅=-4·15+13=-43
S₁₅=(a₁+a₁₅)·15/2=(9+(-43))·15/2=255
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Verified answer
a₁=-4·1+13=9
a₂=-4·2+13=5
d=a₂-a₁=5-9=-4
a₁₅=-4·15+13=-43
S₁₅=(a₁+a₁₅)·15/2=(9+(-43))·15/2=255