1.8, Problem 37: A lidless cardboard box is to be made with a volume of 4 m3
. Find the
dimensions of the box that requires the least amount of cardboard.
Solution: If the dimensions of our box are x, y, and z, then we’re seeking to minimize
A(x, y, z) = xy + 2xz + 2yz subject to the constraint that xyz = 4. Our first step is to make
the first function a function of just 2 variables. From xyz = 4, we see z = 4/xy, and if we substitute
this into A(x, y, z), we obtain a new function A(x, y) = xy + 8/y + 8/x. Since we’re optimizing
something, we want to calculate the critical points, which occur when Ax = Ay = 0 or either Ax
or Ay is undefined. If Ax or Ay is undefined, then x = 0 or y = 0, which means xyz = 4 can’t
hold. So, we calculate when Ax = 0 = Ay. Ax = y − 8/x2 = 0 and Ay = x − 8/y2 = 0. From
these, we obtain x
2y = 8 = xy2
. This forces x = y = 2, which forces z = 1. Calculating second
derivatives and applying the second derivative test, we see that (x, y) = (2, 2) is a local minimum
for A(x, y). To show it’s an absolute minimum, first notice that A(x, y) is defined for all choices
of x and y that are positive (if x and y are arbitrarily large, you can still make z REALLY small
so that xyz = 4 still). Therefore, the domain is NOT a closed and bounded region (it’s neither
closed nor bounded), so you can’t apply the Extreme Value Theorem. However, you can salvage
something: observe what happens to A(x, y) as x → 0, as y → 0, as x → ∞, and y → ∞. In each
of these cases, at least one of the variables must go to ∞, meaning that A(x, y) goes to ∞. Thus,
moving away from (2, 2) forces A(x, y) to increase, and so (2, 2) is an absolute minimum for A(x, y).
We convert degrees to radian by multiplying by (
)
= (345) × ( 
)
= (
) × (π)
= 1.9166π
= 1.9166 × 3.14
= 6.018 rad
≈ 6.02 rad
The surface area of the triangular prism is 144 cm²
<u>Explanation:</u>
Given:
Base of the triangle = 4
Height of the triangle = 3
Hypotenuse of the triangle = 5
Length of the prism = 11
Surface area of the rectangular prism = ?
Surface area, A = 
On substituting the value, we get:
Therefore, the surface area of the triangular prism is 144 cm²
(x,y)
x=input
y=output
example
we see
g=(1,2)
theefor
g(1)=2
a.
f(4)=1
g(1)=2
g(f(4))=2
b.
g(-2)=4
f(4)=1
f(g(-2))=1
c.
f(3)=5
g(5)=5
f(5)=0
f(g(f(3)))=0
