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Question
Choose the correct alternative answer for the following question .
15, 10, 5,... In this A.P sum of first 10 terms is...
Options
-75
-125
75
125
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Solution
The given sequence is 15, 10, 5,...
Here,
a = 15
d = –5
We know that,
\[S_n = \frac{n}{2}\left( 2a + \left( n - 1 \right)d \right)\]
\[ S_{10} = \frac{10}{2}\left( 2a + \left( 10 - 1 \right)d \right)\]
\[ = 5\left( 2\left( 15 \right) + 9\left( - 5 \right) \right)\]
\[ = 5\left( 30 - 45 \right)\]
\[ = 5\left( - 15 \right)\]
\[ = - 75\]
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