Answer:
Length = 15.14, width = 7.14 and height = 2.43 inches.
( correct to the nearest hundredth).
Step-by-step explanation:
Let the lengths of the squares cut out be x inches.
Then the width of the box will be 12 - 2x and the length will be
20 - 2x inches.
The height of the box is x inches.
Volume V = x(20-2x)(12-2x)
To find the value of x when V is a maximum we find the derivative and equate it to zero.
V = x( 240 - 64x + 4x^2)
V = 4x^3 - 64x^2 + 240x
dV/dx = 12x^2 - 128x + 240 = 0
4(3x^2 - 32x + 60) = 0
This won't factor so we use the formula:
x = [-(-32) +/- √((-32)^2 - 4*3*60)] / 6
= 8.24, 2.43.
So one of these gives a maximum Volume.
The second derivative
d^2V/dx^2 = 24x - 128
When x = 8.24, this = 69.6 (positive) so this gives a minimum.
x = 2.43 gives a negative value ( -49.7) so this is the maximum
So the dimensions are:-
length = 20 - 2(2.43) = 15.14
width = 12 - 2(2.43) = 7.14
height = 2.43.
