Question

the third term of an ap is 15 and the sum of first 10 terms is 125. find the first term and the common difference​

Answer 1

a
3
​
=15,s
10
​
=125

a
3
​
=a+(3−1)d

15=a+2d

a=15−2d

s
10
​
=
2
10
​
[2a+(10−1)d]

125=5[2a+9d]

25=2a+9d
25=2(15−2d)+9d
25=30−4d+9d
5d=−5
d=−1
a=15−2×(−1)
a=17
a
10
​
=a+(10−1)(−1)
a
10
​
=17−9
a
10
​
=