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Another proof without words, at the suggestion of Semiclassical:

The dark rectangle has some fixed diagonal $d$. The large square has area $d^2$.

A simple explanation without proof or pictures:

The diagonal of a rectangle is at least as long as each of its sides, so the square of the diagonal must be at least the product of the sides.

In fact, the squared diagonal must be at least twice the area, i.e. $a^2+b^2ge 2ab$ if orthogonal sides' lengths are $a,,b$. Why? Because the difference is $(a-b)^2ge 0$.

You can prove that no such rectangle exists as follows:

Let $l ge b$ (one side of the rectangle has to be the longest). Since $2l+2b=72$ you have $2lge l+b= 36$ so $l ge 18$

Then the diagonal of a right-angled triangle is the longest side so $dgt lge 18$ for a non-degenerate triangle, and the only degenerate case which arises is with $l=36, b=0, d=36$.

The answer given, though arithmetically correct does not represent a real wall.

I am not sure what the question means, though, as it is curiously phrased. The question asks for "the area of the wall" and not "the area of the rectangle bounded by the wall" and had the answer not been set out, I might have been thinking of a wall of uniform thickness and external perimeter $72$ and an internal diagonal of $18$ to make any sense of it.

No. As others have said.

What this looks like to me (as someone who has taught HS Chem & Physics for years and has helped write middle school math content) is a question written trying to get someone to put together the solution as shown, but without checking whether the numbers given make any real-world sense. I have certainly made this mistake myself, even though I try really hard to catch it.

If this is a standardized test question (or one from a textbook, practice book, online resource, etc.), fair play, we've caught a poorly written question.

If this is a question you are writing yourself, and you want to improve it, you could change the parameters this way:

Total perimeter: 70
Diagonal: 25 (I don't think you'll find any nice whole numbers - aka pythagorean triples - using a perimeter of 72.)

This should now give the solution of:

• $I^2 + B^2 = 25^2 = 625 $


• $2I + 2B = 70 $


• $I + B = 35 $


• $I^2 + 2IB + B^2 = 1,225 $


• $2IB = 600 $


• 
  $IB = 300$ ,

which makes sense, given that I used a (3,4,5) right triangle (scaled by 5) in my setup. (Which means that I = 15 and B = 20, for a hypotenuse of 25.)

Hope that helps!

-Van

Another PWW (noted by AlexanderJ93 and others):

$hspace{5cm}$

No. Using Pythagoras and a simple inequality we get
$$d^2=a^2+b^2geq 2abgeq ab$$
If $a,b$ are the sides and $d$ the diagonal

No, use the Pythagorean Theorem.

$$c^2 = a^2+b^2$$

$c$ is the length of the diagonal. It divides the rectangle into two congruent right triangles with hypotenuse $c$. $a$ and $b$ are the pairs of sides of the rectangle (and the other two sides of each congruent right triangle).

Recall for any real number, its square must be non-negative.

$$(a-b)^2 geq 0 implies a^2-2ab+b^2 geq 0 implies color{blue}{a^2+b^2 geq 2ab}$$

The area of the rectangle is $ab$, but $c^2 geq 2ab$, so the square of the diagonal is at least twice the area of the rectangle.

Now, to find the area itself.

For the diagonal:

$$c^2 = a^2+b^2$$

$$implies 18^2 = a^2+b^2$$

$$color{blue}{324 = a^2+b^2} tag{1}$$

For the perimeter:

$$2(a+b) = 72$$

$$a+b = 36$$

Now, define one variable in terms of the other.

$$color{purple}{a = 36-b} tag{2}$$

Combine $(1)$ and $(2)$.

$$324 = a^2+b^2 implies 324 = (36-b)^2+b^2$$

$$324 = 36^2-2(36)b+b^2+b^2 implies 324 = 1296-72b+2b^2 implies 2b^2-72b+972 = 0$$

But $$Delta = b^2-4ac$$

$$Delta = 72^2-4(2)(972) = -2592$$

$$implies Delta < 0$$

Thus, there is no solution. (No such rectangle exists.)

Adjusted the PWW given by farruhota (and ripping off their very image) to improve by a factor of $2$ (cf. also Théophile's answer):

$A=lw$

$P=2(l+w)$

$d=sqrt{l^2+w^2}$

Can $A>d^2$?

Can $lw>l^2+w^2$?

$-lw>l^2-2wl+w^2=(l-w)^2$

Width and length are necessarily positive. The square of their difference also must be positive.

So we have a negative number that must be greater than a positive number. A contradiction.

A wall has a thickness.

Let's say the rectangle of the wall has width a and length b on the outside, so 2a + 2b = 72 or a + b = 36.

Assume the wall has a uniform thickness t. Then we can calculate the length of the diagonal as $(a - 2t)^2 + (b - 2t)^2 = d^2$. Given that d = 18, this lets us calculate t based on d, and the area of the wall is (2a + 2b - 4t) * t or (72 - 4t) * t.

The solution is $t = 9 ± (9a - a^2/4 - 40.5)^{1/2}$. For a solution to exist, we need $9a - a^2/4 - 40.5 ≥ 0$ or $162^{1/2} ≥ |a - 18|$ or $18 - 162^{1/2} ≤ a ≤18 + 162^{1/2}$, so a is roughly between 5 and 31.

We can exclude the solution $t = 9 + (9a - a^2/4 - 40.5)^{1/2}$ - the walls cannot be more than 9 thick. Therefore $t = 9 - (9a - a^2/4 - 40.5)^{1/2}$. We can show that the calculated thickness is always ≥ 0.

This t can be used to calculate the area of the wall as (72 - 4t) * t.

Alternatively, if the wall has zero thickness, then the diagonal is not 18. Everything follows from a false statement, so the wall can have any area.