Question

determine o número de termos da PA(-24,-36,-48,...-252) e a soma desses termos

Answer 1

P.A(-24,-36,-48,-60,-72,-84,-96,-108,-120,-132,-144,-156,-168,-180,-192,-204,-216,-228,-240,-252)
n = 20
r = a 2 - a 1
r = -36  - (-24)
r = -36 + 24
r = -12
a n = -252
a n = a 1 + (n - 1 ).r
-252 = -24 + (n - 1 ).-12
-252 = -24 + 12 - 12 n
-252 = -12 - 12 n
-252 + 12 = -12 n
-240 = -12 n
-12 n = -240
n =  -240/-12
n = 20

S n = n.(a 1 + a n) / 2
S 20 = 20 . (-24 + (-252) / 2
S 20 = 20 .(-24 - 252 ) / 2
S 20 = 20 .(-276) / 2
S 20 = -5520/2
S 20 = -2760