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U₉ = 51
Dit : S₁₅ = ...?
b = 51-16 / 9-4
= 35/5
= 7
U₁ = U₄ - 3b
= 16 - 3(7)
= 16-21
= -5
Sn = n/2 (2a+(n-1)b)
S₁₅ = 15/2 (2(-5)+(15-1)7)
= 15/2 (-10 + 98)
= 15/2 (88)
= 15(44)
= 660