Question

If the sum of two extreme numbers of an A.P. with four terms is 8 and product of remaining two middle term is 15, then greatest number of the series will be.

• A. 5
• B. 7
• C. 9
• D. 11

Answer 1

Answer: (B)

7

Answer 2

Let four numbers are $a - 3 d , a - d , a + d , a + 3 d$ .

Now $( a - 3 d ) + ( a + 3 d ) = 8$ $\Rightarrow$ $a = 4$ and $( a - d ) ( a + d ) = 15$ $\Rightarrow$ $a ^ { 2 } - d ^ { 2 } = 15$ $\Rightarrow$ $d = 1$

Thus required numbers are 1, 3, 5, 7.

Hence greatest number is 7.