Question

The 6th term of an AP is -10 and the 10th term is -20. Determine the 15th term of the ap.

Answer 1

6th term is 10
that means a+5d=10----(i)
and 10th term is 20
that means a+9d=20(ii)
now subtract the equation 1st and 2nd
a+5d=10
a+9d=20
- - -
-------------------
-4d=-10
d= -10/-4
d=5/2
now put the value of d in equation 1st
a+5d=10
a+5×5/2=10
a+25/2=10
a=10-25/2
=(20-25)/2
= -5/2
so 15th term is
a+14d
= -5/2+14×5/2
=-5/2+35
= (-5+70)/2
=65/2