Question

the 24th term of an AP in twice,its 10th term. show that its 72nd term is 4 times its 15th term.

Answer 1

a+23d=2(a+9d)
a+23d=2a+18d
a-5d=0
A=5d

4(a+14d)
4a+56d=t14
4(5d)+56d
20d+56d=76d

Also,t72=a+71d
5d+71d=76d

So t72=4t15(hence proved)