The first term of an A.P. of 30 non-negative terms is 103. If the sum of this A.P. is the cube of its last term, then its common difference is:
- 587
- 2583
- 1529
- 529
Answer
(1) 587
Solution
Let the last term be l. Then S30= 302 ( 103+l )=50+15l
Given S30=l3 l3-15l-50=0
l=5 satisfies it: 125-75-50=0 (l-5)(l2+5l+10)=0
l2+5l+10=0 has no real root (discriminant 25-40<0) l=5
103+29d=5 29d= 53
d= 587 (all terms then lie between 103 and 5, so they are non-negative)
Answer: (1)