Question

determine o número de termos da P.A. (-6,-9,-12, ... , -66)

Answer 1

a3 = a1 + 2r

-12 = -6 + 2r

-6= 2r

r = -6/2
r = 3


Logo;


-6,-9,-12,-15,-18,-21,-24,-27,-30,-33,-36,-39,-42,-45,-48,-51,-54,-57-60,-63,-66

ou seja 21 termos

Answer 2

vamos lá...

P.A.(-6,-9,-12,...,-66)

logo

an=-66        a₁=-6       r=-9-(-6)=-9+3=-3     n=n° de termos

an=a₁+(n-1)r
-66=-6+(n-1)(-3)
-66=-6-3n+3
-66=-3-3n
-66+3=-3n
3n=63
n=63÷3
n=21

Logo a P.A. tem 21 termos