Question

Encontre a soma dos termos :
(300,280,260...)15°

Answer 1

an = a1 + (n -1). r
an = 300 + (15-1).-20
an = 300 + 14.-20
an = 300 + (-280)
an = 300 - 280
an = 20

300+280+260+240+220+200+180+160+140+120+100+80+60+40+20 = 2400

a1+a2+a3+a4+a5+a6+a7+a8+a9+a10+a11+a12+a13+a14+a15 = 2400