5 ## THE ISOSCELES RIGHT TRIANGLEAN ISOSCELES RIGHT TRIANGLE is one of two special triangles. (The other is the 30°-60°-90° triangle.) The student should know the ratios of the sides. (An isosceles triangle has two equal sides. See Definition 8 in Some Theorems of Plane Geometry. The theorems cited below will be found there.) Theorem.
To find the ratio number of the hypotenuse
Therefore,
(Lesson 26 of Algebra.) Therefore the three sides are in the ratio 1 : 1 : . Note that since the right triangle is isosceles, then the angles at the base are equal. (Theorem 3.) Therefore each of those acute angles is 45°. (For the definition of measuring angles by "degrees," see Topic 12.) Example 1. Evaluate sin 45° and tan 45°.
We see:
on rationalizing the denominator. (Lesson 26 of Algebra.)
Problem. Evaluate cos 45° and csc 45°.
Thus cos 45° is equal to sin 45°; they are complements.
Example 2. Solve the isosceles right triangle whose side is 6.5 cm.
But in every isosceles right triangle, the sides are in the ratio 1 : 1 : , as shown on the right. In the triangle on the left, the side corresponding to 1 has been multiplied by 6.5. Therefore every side will be multiplied by 6.5. The hypotenuse will be 6.5. (The theorem of the same multiple.) Whenever we know the ratio numbers, we use this method of similar figures to solve the triangle, and not the trigonometric Table. (In Topic 6, we will solve right triangles the ratios of whose sides we do not know.) Example 3. In an isosceles right triangle, the hypotenuse is inches. How long are the sides?
How has the side corresponding to been multiplied? According to the rule for multiplying radicals, it has been multiplied by . Therefore, all the sides will be multiplied by . And 1 = . Next Topic: Solving Right Triangles Please make a donation to keep TheMathPage online. Copyright © 2014 Lawrence Spector Questions or comments? E-mail: themathpage@nyc.rr.com |