The growth rate y in pounds per month of an infant is relate

The growth rate y (in pounds per month) of an infant is related to present weight x (in pounds) by the formula y = cx(21 - x), where c is a positive constant and 0

Solution

y = cx(21 - x)

y = 21cx - x^2

y\' = 21c - 2cx

y\'\' = 0 - 2c = -2c

Now the maximum/minimum value will be when y\' = 0

21c - 2cx = 0

x = 10.5

since y\'\' is negative, at x = 10.5, weight will be maximum.