Question

Quantos termos tem a Progressão Aritmética (15, 5, ..., -5.005) ?

Answer 1

$\boxed{a_n=a_1+(n-1)*r}\\\\ a_n=-5005\\ a_1=15\\ r=5-15\ \rightarrow\ r=-10\\\\ -5005=15+(n-1)*-10\\\\ -5005-15=-10(n-1)\\\\ -5020=-10(n-1)\\\\ (n-1)=\frac{-5020}{-10}\\\\ n-1=502\\\\ n=502+1\\\\ \boxed{n=503}$