Question

calcule a razão deuma .p.a. sabendo se que a1=100 e a 21=-40

Answer 1

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$a_n=a_1+(n-1)r \\ \\ -40=100+(21-1)R \\ \\ -40=100+20r \\ \\ 20r=-40-100 \\ \\ 20r=-140 \\ \\ r=-140\div20 \\ \\ r=-7$