Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
4. Inscribed Angles and Polygons
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Exercise 15 Page 558

Start with finding the measures of ∠ J and ∠ K using the Measure of an Inscribed Angle Theorem.

a=20 and b=22

Practice makes perfect

To find the values of a and b, let's analyze the given diagram.

First, we need to find the measures of ∠ J and ∠ K. To do this we will use the Measure of an Inscribed Angle Theorem, which states the following.

Measure of an Inscribed Angle Theorem

The measure of an inscribed angle is one-half the measure of its intercepted arc.

From the diagram, we can see that ∠ J intercepts arc MK.

This arc consists of two smaller arcs — ML and LK. Hence, adding their measures, we can calculate the measure of MK. mMK&=mML+mLK & ⇓ mMK&= 130^(∘)+ 110^(∘)=240^(∘) According to the stated theorem, the measure of ∠ J is half the measure of its intercepted arc MK. m∠ J=mMK/2=240^(∘)/2=120^(∘) Similarly, we can find the measure of ∠ K. This angle intercepts arc JL, which consists of JM and ML.

Thus, by adding the given measures of JM and ML, we can find the measure of JL. mJL&=mJM+mML & ⇓ mJL&= 54^(∘)+ 130^(∘)=184^(∘) Finally, dividing the measure of mJL by 2, we can calculate the measure of m∠ K. m∠ K=mJL/2=184^(∘)/2=92^(∘) Now, since JKLM is an inscribed quadrilateral, we can apply the Inscribed Quadrilateral Theorem. Let's recall what it states!

Inscribed Quadrilateral Theorem

A quadrilateral can be inscribed in a circle if and only if its opposite angles are supplementary.

According to this theorem, opposite angles ∠ J and ∠ L, as well as ∠ K and ∠ M, are supplementary. This means that the sum of their measures is equal to 180^(∘). m∠ J+m∠ L=180^(∘) m∠ K+m∠ M=180^(∘) Let's substitute the measures of these angles and solve each equation for the unknown variable. We will start with the first equation!
m∠ J+m∠ L=180^(∘)
120^(∘)+ 3a^(∘)=180^(∘)
3a^(∘)=60^(∘)
a^(∘)=20^(∘)
Now, let's solve the second equation.
m∠ K+m∠ M=180^(∘)
92^(∘)+ 4b^(∘)=180^(∘)
4b^(∘)=88^(∘)
b^(∘)=22^(∘)
Therefore, the value of a is 20 and the value of b is 22.