Question

calcule a soma dos multiplos positivo de 9 menos que 100

Answer 1

a 1 = 9
a n = 99
r = 9 
99 =9-9+9n
99=9n
n=99/9
n=11
S n = n.(a1+an)/2
S 11 = 11(108)/2
S11 = 1188/2
S 11 = 594

Answer 2

a 1 = 9
a n = 99
r = 9
a n = a 1 + (n - 1 ).r
99 = 9 + (n - 1 ).9
99 = 9 - 9 + 9 n
99 = 0 + 9 n
99 = 9 n
99/9 = n
n = 11
S n = n.(a 1 + a n ) / 2
S 11 = 11 . (9 + 99) / 2
S 11 = 11 (108) / 2
S 11 = 1188 / 2
S 11 = 594