Обчислити -25/2-71/6-67/6-...-5/2

Question

Обчислити -25/2-71/6-67/6-...-5/2

Answer 1

-25/2-71/6-67/6-...-5/2

a1= -25/2
a2= -71/6
d=a2-a1 = -71/6+25/2 = 2/3

an = a1+d(n-1) = - 5/2
-12,5 + 2/3(n-1) = - 2,5
2/3n = -2,5+12,5+2/3
2/3n = 32/3
2n = 32
n = 16

S16 = (a1+a16)/2*16 = (-12,5-2,5)/2*16 = -7,5 *16 = -120