Question

If S = smiddle" class="latex-formula">(a + L), find n when, S= 640, a = 25 and L = 39

Answer 1

Answer:

n = 20

Step-by-step explanation:

Sum of an arithmetic series

$S_n=\dfrac{n}{2}(a+l)$

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:

$\implies 640=\dfrac{n}{2}(25+39)$

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

$\implies 1280=n(25+39)$

$\implies 1280=64n$

$\implies n=\dfrac{1280}{64}$

$\implies n=20$