Question

Quantos termos tem a Progressão Aritmética (20, 25, ..., 5.005) ?
(A) ( ) 5000
(B) ( ) 1000
(C) ( ) 999
(D) ( ) 998
(E) ( ) 997

Answer 1

.
$a_n=5.005 \\ a_1=20 \\ r=25-20=5 \\ n=? \\ \\ a_n=a_1+(n-1)r \\ \\ 5.005=20+(n-1)5 \\ \\ 5.005=20+5n-5 \\ \\ 5.005=15+5n \\ \\ 5n=5.005-15 \\ \\ 5n=4990 \\ \\ n=4990\div5 \\ \\ n=998 \\ \\ \\ \\ Letra D$