Given two numbers x and y such that:
x + y = 12 ... (1)
from equation (1)
y = 12 - x
Using this value of y, we represent xy as
xy = f(x)= x(12 - x)
f(x) = 12x - x^2
Differentiating the above function:
f'(x) = 12 - 2x
Maximum value of f(x) occurs at point for which f'(x) = 0.
Equating f'(x) to 0 we get:
12 - 2x = 0
2x = 12
> x = 12/2 = 6
Substituting this value of x in equation (2):
y = 12 - 6 = 6
Therefore, value of xy is maximum when:
x = 6 and y = 6
The maximum value of xy = 6*6 = 36
