Question

Numa P.A, calcule o numero de termos sabendo que a razão é -4, o ultimo termo -2 e o primeiro 18

Answer 1

$\large\boxed{\begin{array}{l}\sf a_n=a_1+(n-1)\cdot r\longrightarrow n=\dfrac{a_n-a_1}{r}+1\\\sf a_n=-2\\\sf r=-4\\\sf a_1=18\\\sf n=?\\\sf n=\dfrac{-2-18}{-4}+1\\\\\sf n=\dfrac{-20}{-4}+1\\\\\sf n=5+1\\\sf n=6\checkmark\end{array}}$