Question

7) Ache a1 numa P.A., sabendo que r=1/4 e a17=21.

Answer 1

a₁₇ = 21

r = 1/4 ou 0,25

21 = a₁ + (17 - 1) . 1/4 ---- 21 = a₁ + 16 . 1/4 ----- 21 = a₁ + 4 --- a₁ = 21 - 4

a₁ = 17

Answer 2

an=>a17=>21
a1=>?
n=>17
r=>1/4

an=a1+(n-1)*r


21=a1+(17-1)*1/4

21=a1+16*1/4

21=a1+16/4

21=a1+4

-a1=4-21*(-1)

a1= -4 +21

a1= 17