Fist : 3
then
second : x (-3) = - 9
third : x (-3) = 27
x (-3) = - 81
the 15 term ?
first one = 3 x (-3)⁰
second one = 3 x (-3)¹
third one = 3 x (-3)²
so n one = 3 x (-3)ⁿ⁻¹
then the 15th one = 3 x (-3)¹⁴
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