math precalculus 12

Question

If an Arithmetic series has t₂ = -12.5 and t₄ = -10.7, the determine S₁₅.

Answer 1

If t₂ = the second term = -12.5 and t₄ = -10.7, since it's an arithmetic series, we can deduce that the common difference between terms is 1.8/2 = 0.9.

This means the first term, t₁ = -12.5 - (0.9) = -13.4

So the nth partial sum, S₁₅ = (15/2)[2(-13.4) + (14)(0.9)] = -106.5

Answer 2

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