Question

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

Answer 1

r= a2-a1
r= -9-(-6)
r= -9+6
r= -3

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

Answer 2

an=a1+(n-1).r         r=-9-(-6)= -9+6=-3

-66=-6+(n-1).(-3)-66=-6-3n+3
-6-3n+3=-66
-3n=-66+6-3
-3n=-63 (-1)
3n=63
n=63/3
n=21 termos