High School Math : How to find the length of the hypotenuse of a 45/45/90 right isosceles triangle : Pythagorean Theorem

Study concepts, example questions & explanations for High School Math

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Example Questions

Example Question #20 : Isosceles Triangles

Isosceles

In an isosceles right triangle, two sides equal \(\displaystyle x\). Find the length of side \(\displaystyle h\)

Possible Answers:

\(\displaystyle \sqrt{2}\)

\(\displaystyle 2\sqrt{2}\)

\(\displaystyle \sqrt{x}\)

\(\displaystyle x\)

\(\displaystyle x\sqrt{2}\)

Correct answer:

\(\displaystyle x\sqrt{2}\)

Explanation:

This problem represents the definition of the side lengths of an isosceles right triangle.  By definition the sides equal \(\displaystyle x\)\(\displaystyle x\), and \(\displaystyle x\sqrt{2}\).  However, if you did not remember this definition one can also find the length of the side using the Pythagorean theorem \(\displaystyle (a^{2}+b^{2}=c^{2})\).

\(\displaystyle h^{2}=x^{2}+x^{2}\)

\(\displaystyle h^{2}=2x^{2}\)

\(\displaystyle h=\sqrt{2x^{2}}\)

\(\displaystyle \rightarrow x\sqrt{2}\)

Example Question #1 : 45/45/90 Right Isosceles Triangles

ABCD is a square whose side is \(\displaystyle 8\) units.  Find the length of diagonal AC.

Possible Answers:

\(\displaystyle 16\)

 \(\displaystyle 8\sqrt{3}\)

\(\displaystyle 12\)

none of the other answers

\(\displaystyle 8 \sqrt{2}\)

Correct answer:

\(\displaystyle 8 \sqrt{2}\)

Explanation:

To find the length of the diagonal, given two sides of the square, we can create two equal triangles from the square. The diagonal line splits the right angles of the square in half, creating two triangles with the angles of \(\displaystyle 45\), \(\displaystyle 45\), and \(\displaystyle 90\) degrees. This type of triangle is a special right triangle, with the relationship between the side opposite the \(\displaystyle 45\) degree angles serving as x, and the side opposite the \(\displaystyle 90\) degree angle serving as \(\displaystyle x\sqrt{2}\).

 

Appyling this, if we plug \(\displaystyle 8\) in for \(\displaystyle x\) we get that the side opposite the right angle (aka the diagonal) is \(\displaystyle 8\sqrt{2}\)

Example Question #2 : 45/45/90 Right Isosceles Triangles

The area of a square is \(\displaystyle 144\). Find the length of the diagonal of the square.

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle 4\sqrt{2}\)

\(\displaystyle 16\sqrt{2}\)

\(\displaystyle 16\)

\(\displaystyle 12\sqrt{2}\)

Correct answer:

\(\displaystyle 12\sqrt{2}\)

Explanation:

If the area of the square is \(\displaystyle 144\), we know that each side of the square is \(\displaystyle 12\), because the area of a square is \(\displaystyle s^{2}\).

Then, the diagonal creates two \(\displaystyle 45/45/90\)special right triangles. Knowing that the sides = \(\displaystyle 12\), we can find that the hypotenuse (aka diagonal) is \(\displaystyle 12\sqrt{2}\)

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