Inequality(Maximum Value)

[vishu1414]

Hi
Please help me to approach the problem :
If a<=5 ,b <=3 ,and ab=12 ,What is maximum possible value of (a+b)?
A)23/5
B)34/5
C)7
D)37/5
E)9

Ans:D

[ikaplan]

This question seems strange to me. Can you please state the source?

Thanx!

[finites]

answer c is possible..

[goalevan]

The product of two numbers (a and b) under a sum constraint (a + b < x) is maximized when a = b.
But the sum of numbers under a product constraint is maximized when these numbers DO NOT equal each other. An extreme example is (1/10000)*(120000) = 12, for which the sum is greater than 120,000.

To solve, we can maximize one of the numbers and minimize the other, given the constraints.

a <= 5 and b <= 3, so we can set a = 5 and b = 12/5 = 2.4.

a + b is then 25/5 + 12/5 = 37/5, while satisfying the constraints ab = 12, a <= 5, and b <=3.

[kevincanspain]

Hi
Please help me to approach the problem :
If a<=5 ,b <=3 ,and ab=12 ,What is maximum possible value of (a+b)?
A)23/5
B)34/5
C)7
D)37/5
E)9

Ans:D

The two numbers are a and 12/a, where a is between 4 and 5 inclusive

The sum is a + 12/a , which increases as a increases from 4 to 5 (12/a decreases from 3 to 2.4, a change of only 0.6, not enough to offset the increase in a)

When a=5, a + 12/a = 5 + 12/5 = 37/5