Find the sum of last ten terms of the A.P.: $8, 10, 12, 14,…, 126$.
Given:
Given A.P. is $8, 10, 12, 14,…, 126$.
To do:
We have to find the sum of the last ten terms of the A.P.: $8, 10, 12, 14,…, 126$.
Solution:
For finding the sum of the last ten terms, we can write the given A.P. in reverse order.
This implies, the A.P. now becomes,
$126, 124,........, 14, 12, 10, 8$
Here,
First term \( (a)=126, \) common difference \( (d)=124-126=-2 \)
We know that,
${S}_{n}=\frac{n}{2}[2 a+(n-1) d]$
$\therefore \mathrm{S}_{10}=\frac{10}{2}[2 a+(10-1) d]$
$= 5[2(126)+9(-2)]$
$=5(252-18)$
$=5 \times 234$
$=1170$
The sum of the last ten terms of the A.P.: $8, 10, 12, 14,…, 126$ is $1170$.
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