Marieleslielegviz
Answered

A stack of telephone poles has 30 poles in the bottom row. There are 29 poles in the second row, 28 in the next row and son on. How many poles are in the stack if there are 5 poles in the top row?

Sagot :

n = ?

an = 5

a₁ = 30

d = -1

Find n:

5 = 30 + (n-1)(-1)

5 = 30 + 1 -n

n = 31-5

n = 26

Find the sum of the poles:

S₃₄ = 26/2 {30 + 5)

S₃₄ = 13 (35)

S₃₄ = 455

ANSWER: 455 poles