Question

Determine o número de termos da P.A (-6,-9,-12.....,60) ?

Answer 1

P.A.(-6,-9,-12,.....60)
r = a 2 - a 1
r = -9 + 6
r = - 3
a n = a 1 + (n - 1 ).r
60 = -6 + (n - 1 ).-3
60 = -6 + 3 - 3 n
60 = -3 - 3 n
60 + 3 = - 3 n
63 = - 3 n
-3 n = 63
n = 63/-3
n = 21