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Question

If a3=15,S10=125, find d and a10

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Solution

Given: a3=15,S10=125
a3=15
a+2d=15
a=152d ------------- (1)
Now,
s10=125
125=102[2a+(n1)d]
125=5[2(152d)+(101)d] [Using eq.(1)]
1255=304d+9d
25=30+5d
2530=5d
5=5d
d=1
From (1), we have

a=152×1=17

Hence, a10=a+9d=179=8


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