Question

Determine o 21° da P.A (1,-1)​

Answer 1

Resposta:

$a_n=a_{21}=?\\ a_1=1\\ n=21\\ r=-1-1=-2\\ \\ Termo~~geral\\ \\ a_n=a_1+(n-1)r\\ \\ a_{21}=1+(21-1)(-2)\\ \\ a_{21}=1+(20)(-2)\\ \\ a_{21}=1-40\\ \\ a_{21}=-39$