Question

Given that a window entails a rectangle capped by a semi-circle, given that the semi-circle’s diameter concides with the rectangle’s width, given that the window’s outside (linear and curvilinear) perimeter is 24 feet, and given that the semi-circle’s stained glass transmits half the light of the rectangle’s unstained glass, determine the window’s rectangular and circular dimensions that will maximize the light transmitted.

Answers

Let the width of the rectangle be 2x , making the radius of the semicircle = x
let the height of the rectangle be y
then 2x + 2y + 2πx = 24
x + y + πx = 12
y = 12-x-πx

Assume that the amount of light (L) is a function of the area
L= area of rectangle + (1/2) area of semicircle
= 2xy + (1/2)(1/2)π x^2
= 2x(12-x-πx) + (1/4)π x^2 = 24x - 2x^2 - 2πx^2 + (1/4)π x^2
dL/dx = 24 - 4x - 4πx + (1/2)πx
= 0 for a max of L

times 2
48 - 8x - 8πx + πx = 0
48 = x(8 + 8π - π)
x = 48/(8+7π) = appr1.6
then r = 5.37

State the conclusion
(check my arithmetic, I should have written it out on paper)
A D C C C

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