Two non-negative numbers whose sum is 9 and the product of the one number and square of the other number is maximum, are:
Explanation
Solution
Let the numbers be A and B.
Given: A+B=9⇒A=9−B
Let the function to maximize be f=AB2.
f=(9−B)B2=9B2−B3
For maxima, the first derivative must be zero:
f′=dBd(9B2−B3)=18B−3B2=0
3B(6−B)=0
So, B=0 or B=6.
To check for maximum, we find the second derivative:
f′′=dBd(18B−3B2)=18−6B
At B=6:
f′′=18−6(6)=18−36=−18
Since f'' < 0 at B=6, the function is maximum.
Finding A:
A=9−6=3
The numbers are 3 and 6.
Correct Option: 2
Explanation
Solution
Let the numbers be A and B.
Given: A+B=9⇒A=9−B
Let the function to maximize be f=AB2.
f=(9−B)B2=9B2−B3
For maxima, the first derivative must be zero:
f′=dBd(9B2−B3)=18B−3B2=0
3B(6−B)=0
So, B=0 or B=6.
To check for maximum, we find the second derivative:
f′′=dBd(18B−3B2)=18−6B
At B=6:
f′′=18−6(6)=18−36=−18
Since f'' < 0 at B=6, the function is maximum.
Finding A:
A=9−6=3
The numbers are 3 and 6.
Correct Option: 2