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).
0.65 is the correct one if not up there try 0.45 other possible one
Step-by-step explanation:
A rational number is defined as any number which can be expressed in fractional p/q of two integers, where 'p' is the numerator and 'q' is the denominator. 'q' can never be zero. A rational number can also be expressed as a terminating decimal. So, when 0.25 is added in 0.45 it becomes 0.65 which is rational.
We know that the ocean floor has a depth of 247 ft, and we also know that the diver is<span> underwater at depth of 138 ft, so its distance from the ocean floor will be:
</span>
ft
<span>
Now, the </span>rock formations rises to a peak 171 to above the ocean floor, so to find <span>how many feet below the top of the rock formations is the diver, we are going to subtract the distance to the driver form the ocean floor from the rock formations height:
</span>
ft
<span>
We can conclude that the diver is 62 feet </span><span>
below the top of the rock formations.</span>
