Question

qual a s soma dos 30 primeiros termos da pa (-13,-7,-1)?

Answer 1

S 30 = 30.(a 1 + a n) / 2
r = a 2 - a 1
r = -7 + 13
r = 6
a n = a 1 + (n - 1 ).r
a 30 = -13 + (30 - 1 ).r
a 30 = -13 + 29(6)
a 30 = -13 + 174
a 30 = 161
S 30 = 30.(-13 + a n ) / 2
S 30 = 30 . (-13 + 161) / 2
S 30 = 30 . 148 / 2
S 30 = 4440/2
S 30 = 2220