Question

quantos termos tem a (p.A) ( 15,20,25,....160)

Answer 1

 an = a1 + (n-1) .r (fórmula da PA)

160 = 15 + (n-1) .5
160 = 15 + 5n -5
160 = 15 -5 + 5n
160 = 10 + 5n
160-10 = 5n
150 = 5n
n= 150/5
n = 30

Answer 2

Sabemos que:

a_n = a_1 + (n-1)*r

r = 20-15 = 5
a_1 = 15
a_n = 160

160 = 15 + (n-1)*5

160-15 = (n-1)*5

145 = (n-1)*5

(n-1) = 145/5

n-1 = 29

n = 29+1

n = 30