QUESTION

CSAT

Hard

Maths

Prelims 2025

Let p+q=10p + q = 10, where pp and qq are integers.

Value-I = Maximum value of p×qp \times q when pp, qq are positive integers. Value-II = Maximum value of p×qp \times q when p6p \ge -6, q4q \ge -4.

Which one of the following is correct?

Select an option to attempt

Explanation

Given: p+q=10p + q = 10.

The product p×qp \times q is maximum when pp and qq are as close as possible in value.

For Value-I (p, q positive): To get equal or nearly equal integers, p=5p = 5 and q=5q = 5. \Rightarrow Maximum product = 5×5=255 \times 5 = 25.

For Value-II (p6p \geq -6, q4q \geq -4): We still have p+q=10p + q = 10. Thus q=10pq = 10 - p.

Now, p×q=p(10p)=10pp2p \times q = p(10 - p) = 10p - p^2, which is maximized at p=5p = 5, giving product = 2525.

Even when negatives are allowed (p6p \geq -6, q4q \geq -4), the combination (p,q)=(5,5)(p, q) = (5, 5) still satisfies the condition and gives the maximum product. (The negative pair (6,4)(-6, -4) would give 24<2524 < 25.)

Therefore, Value-I = Value-II = 25.

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