Mathematics, 25.01.2021 20:00 NONO3456

Jamal wants to install a rectangular 2025 cm² window in his garden shed. That area of window comes in several length/width combinations, and he wants to minimize the perimeter to prevent drafts. What is the least perimeter possible for the window?

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Answer from: Silkyruthie

The ideal size will be a 45cmx45cm square, giving you a perimeter of 180cm.

Step-by-step explanation:

The shortcut answer here is that the square will always be the most efficient rectangle for border to area, so the sides should be the square root of 2025 cm, or 45 cm each, giving you a total of 180 minimum perimeter.

The full solution is to express the perimeter as a function of the length and width, define either length or width as a function of the other, take the entire expression's derivative and solve for zero. Let's do it:

So first the area:

2025 = wl

l = 2025 / w

Now the sides:

$p = 2w + 2l\\p = 2w + 2(2025 / w)\\p = 2w + 4050 / w$

Then we can take its derivative with respect to w

$p = 2w + 4050\frac{1}{w}\\\frac{dp}{dw} = 2 + 4050\frac{-1}{w^2}\\$

Now we can set the derivative to zero, and solve for w:

$\frac{dp}{dw} = 2 + 4050\frac{-1}{w^2}\\0 = 2 + \frac{-4050}{w^2}\\-2 = \frac{-4050}{w^2}\\2w^2 = 4050\\w^2 = 2025\\w = \sqrt{2025}\\w = 45$

So the width being 45, we can get the length by dividing the area by that amount, which of course is also 45.

Answer from: Quest

something that characterises a right triangle is the fact that we may use pythagoras' theorem to find any one of its sides given that we have the length of the other two sides. here, we have the length of the hypotenuse, √(87) yd, and one other side, √(23) yd, thus we can easily find the length of the remaining side using pythagoras' theorem as defined below:

c^2 = a^2 + b^2, where c is the length of the hypotenuse, and a and b are the lengths of the other two sides.

thus, given that c = √(87) and a = √(23), we may substitute these into the formula we defined above to get:

c^2 = a^2 + b^2

(√87)^2 = (√23)^2 + b^2 (substitute c = √87 and a = √23 into the equation)

87 = 23 + b^2 (evaluate (√87)^2 = 87 and (√23)^2 = 23 - remember that the square root of a number, squared will equal that very same number)

64 = b^2 (subtract 23 from both sides)

√64 = b (take the square root of both sides)

8 = b (evaluate √64 = 8)

therefor, the length of the unknown leg in the right triangle is 8 yd (first answer).