Let the angles be x and (180-x).
Let 2x = 180-x.
Then 3x = 180, and x = 60. One angle is 60 degrees and the other is 180-60, or 120, degrees.
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).
3. 140 m^3
(3 x 6 x 7) + (1 x 2 x 7) = 126 + 14 = 140
4. 216 in^3
(4 x 4 x 9) + (3 x 4 x 6) = 144 + 72 = 216
There are two possibilities, and we don't know which situation is true.
<em>#1).</em> School is in the middle, between home and grandparents.
Home --------- 3/8 ----------- School --------- 2/8 ------- Grandparents .
If this is true, then in the whole day, Tasha walks
(3/8 + 2/8 + 2/8 + 3/8) = 10/8 = <u>1.25 miles</u> .
===========================================
<em>#2):</em> Grandparents house is in the middle, between home and school.
Tasha passes by them on her way to school, and stops there to visit
on her way back home.
Home ------- 1/8 ------- Grandparents ---------- 2/8 ---------- School
If this is true, then in the whole day, Tasha walks
(3/8 + 2/8 + 1/8) = 6/8 = <u>3/4 mile</u> .
