What is the sum of first $10$ terms of the A.P. $15,\ 10,\ 5,\ .......?$
Given: The A.P. $15,\ 10,\ 5,\ .......?$
To do: To find the sum of first $10$ term.
Solution:
$15,\ 10,\ 5,\ ...….$
$a=15\ d=10-15=-5$
$S_n=\frac{n}{2}[2a+( n-1)d]$
$S_10=\frac{10}{2}[2\times15+( 10-1)( -5)]$
$=5[30-5\times9]$
$=5[30-45]$
$=5( -15)=-75$.
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