A square is shown below with its diagonal of length 2.

Each of the triangles formed is an isosceles right triangle with congruent legs - by the 45-45-90 Triangle Theorem, they are 45-45-90 triangles. Also by the 45-45-90 Triangle Theorem, the diagonal, each leg of the triangle has length equal to the hypotenuse divided by . Therefore, the length of a leg - and a side of the square - is 2 divided by , or

.

Rationalize the denominator by multiplying both halves of the fraction by :

Given the length of each side of a square, the area of the square is the square of this length. Therefore, to find the sidelength, take the square root of the area. The area of the square is 49, and , so its sidelength is .

If we are given the area of a square, in order to find the length of a side we must take the square root of the area. We need to know that the formula for area of a square is 

(where s is side) so we plug in 400 for the area and we have

To solve for s we must take the square root of both sides 

and we get 

When taking square roots we reject the negative if we are working with measurements. Our final answer is 20 inches.

The diagonal cuts a square into two right triangles with the legs of the triangle being the sides of the square and the hypotenuse is the diagonal of the triangle.

Using Pythagorean Theorem we get:

or

The Pythagorean Theorem states that the sum of the squares of the sides of the triangle are equal to the square of the hypotenuse, summed up as  but for the case of right isoceles triangles where a and b are equal, it can be rewritten as:

 ,

which simplifies to 

.

The Pythagorean Theorem holds that, in a right triangle, the square of the hypotenuse equals the sum of the squares of the other two sides.  That is,

where  is the hypotenuse and  and  are the other two sides.

Here,  corresponds to , and, since this is a  triangle, the length of  equals the length of , so we know that .

Apply the Pythagorean Theorem.

So the length of the hypotenuse is .

Because the problem states that this the triangle is a right triangle and provides the length of the two legs, the third side (hypotenuse) can be calculated using the Pythagorean Theorem. 

, where a and b can be arbitrarily chosen for either leg length and c represents the length of the hypotenuse. 

 but because the question requires to find the length of the hypotenuse, c cannot be a negative number. Therefore the final answer is +13.9.

45/45/90 triangles are always isosceles. This means that two of the legs of the triangle are congruent. In the figure, it's indicates which two sides are congruent.

From here, we can find the length of the hypotenuse through the Pythagorean Theorem. We can confirm this because the problem has given us no angle measures to perform trig functions with.

c also equals -2√2, but because the hypotenuse is a physical length, the value cannot be negative distance. 

The answer can be found two different ways. The first step is to realize that this is really a triangle question, even though it starts with a square. By drawing the square out and adding the diagonal, you can see that you form two right triangles. Furthermore, the diagonal bisects two ninety-degree angles, thereby making the resulting triangles a  triangle. 

From here you can go one of two ways: using the Pythagorean Theorem to find the diagonal, or recognizing the triangle as a  triangle.

1) Using the Pythagorean Theorem

Once you recognize the right triangle in this question, you can begin to use the Pythagorean Theorem. Remember the formula: , where  and  are the lengths of the legs of the triangle, and  is the length of the triangle's hypotenuse.

In this case, . We can substitute these values into the equation and then solve for , the hypotenuse of the triangle and the diagonal of the square:

The length of the diagonal is .

2) Using Properties of  Triangles

The second approach relies on recognizing a  triangle. Although one could solve this rather easily with Pythagorean Theorem, the following method could be faster.

 triangles have side length ratios of , where  represents the side lengths of the triangle's legs and  represents the length of the hypotenuse.

In this case,  because it is the side length of our square and the triangles formed by the square's diagonal.

Therefore, using the  triangle ratios, we have  for the hypotenuse of our triangle, which is also the diagonal of our square.