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1 vote
If S =
(n)/(2)(a + L), find n when, S= 640, a = 25 and L = 39

User Menepet
by
8.2k points

1 Answer

3 votes

Answer:

n = 20

Explanation:

Sum of an arithmetic series


S_n=(n)/(2)(a+l)

where:

  • a = first term
  • l = last term
  • n = number of terms

Given:


  • S_n=640

  • a=25

  • l=39

Substituting the given values into the formula and solving for n:


\implies 640=(n)/(2)(25+39)


\implies 640 \cdot 2=n(25+39)


\implies 1280=n(25+39)


\implies 1280=64n


\implies n=(1280)/(64)


\implies n=20

User Blingers
by
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