a₃ = a₁ + 2r = 9
a₁₆=a₁ + 15r = 87
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scadem a₁ + 15r - a₁ - 2r = 87 - 9 ; 13r = 78 ; r = 78 :13 = 6
a₁ = 9 -2r = 9 - 12 = - 3
an= a₁ + ( n-1)r = - 3 + ( n -1)· 6 = -3 + 6n - 6 = - 9 + 6n
a₅₁ = a₁ + 50r = - 3 + 50· 6 = 297
S₅₁ =( a₁ + a₅₁ )· 51 / 2 =( - 3 + 297) · 51 / 2 = 51 · 294 :2 =
= 51 · 147 = 7497
