Question

In an AP given a3=15,s10=125,find sand a10​

Answer 1

Step-by-step explanation: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

​  

=26

Answer 2

Step-by-step explanation:

Given:

1. a3=15.
2. S10=125.

To find: A10=?.

a3 => a+2d=15

=> 2(a+2d=15)

=> 2a+4d=30------(1)

S10 => 10/2(2a+9d)=125

=> 2a+9d=25----(2).

By (2)-(1):

=> 2a+9d-(2a+4d}=25-30

=> 2a+9d-2a-4a=-5

=> 5a=-5

=> a=-1-----(3)

Substituting (3) in A3:

=> a+2(-1)=15

=> a=15-2=13.

Formula of A10: a+9d.

=> 13+9(-1)

=> 13-9=4.

A10=4.