Glencoe Math: Course 3, Volume 2
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Glencoe Math: Course 3, Volume 2 View details
2. Geometric Proof
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Exercise 1 Page 382

If two angles are supplementary, then their angle measures add up to 180^(∘).

180^(∘), substitution, 90^(∘), 90 ^(∘)

Practice makes perfect

In the exercise we are given the following figure, including angles 1 and 2.

We are asked to complete the following paragraph proof.

Given m ∠ 1 = m ∠ 2, ∠ 1 and ∠ 2 are supplementary.
Prove ∠ 1 and ∠ 2 are right angles.
Proof m ∠ 1 + m ∠ 2= since they are supplementary angles. Since m ∠ 1 = m ∠ 2, then m ∠ 1 + m ∠ 1=180^(∘) by . Solving the equation gives m ∠ 1 = . Since m ∠ 1 = m ∠ 2, then m∠ 2 is also . Therefore, ∠ 1 and ∠ 2 are right angles.

Let's do it! First, if two angles are supplementary, then by definition their angle measures add up to 180^(∘). We can fill in the first gap.

Given m ∠ 1 = m ∠ 2, ∠ 1 and ∠ 2 are supplementary.
Prove ∠ 1 and ∠ 2 are right angles.
Proof m ∠ 1 + m ∠ 2= 180^(∘) since they are supplementary angles. Since m ∠ 1 = m ∠ 2, then m ∠ 1 + m ∠ 1=180^(∘) by . Solving the equation gives m ∠ 1 = . Since m ∠ 1 = m ∠ 2, then m∠ 2 is also . Therefore, ∠ 1 and ∠ 2 are right angles.

Now, we can rewrite the second sentence of the proof using mathematical symbols. m ∠ 1 = m ∠ 2 ⇒ m ∠ 1 + m ∠ 1=180^(∘) We see that m ∠ 1 was substituted for m ∠ 2 in the expression. The resulting statement is still true because the angle measures are equal.

Given m ∠ 1 = m ∠ 2, ∠ 1 and ∠ 2 are supplementary.
Prove ∠ 1 and ∠ 2 are right angles.
Proof m ∠ 1 + m ∠ 2= 180^(∘) since they are supplementary angles. Since m ∠ 1 = m ∠ 2, then m ∠ 1 + m ∠ 1=180^(∘) by substitution. Solving the equation gives m ∠ 1 = . Since m ∠ 1 = m ∠ 2, then m∠ 2 is also . Therefore, ∠ 1 and ∠ 2 are right angles.
To solve the resulting equation, we should isolate m ∠ 1 on one side of the equation. Let's do it!
m ∠ 1 + m ∠ 1=180^(∘)
2m ∠ 1=180^(∘)
m ∠ 1=90^(∘)
We found that m ∠ 1=90^(∘). Let's write that down.
Given m ∠ 1 = m ∠ 2, ∠ 1 and ∠ 2 are supplementary.
Prove ∠ 1 and ∠ 2 are right angles.
Proof m ∠ 1 + m ∠ 2= 180^(∘) since they are supplementary angles. Since m ∠ 1 = m ∠ 2, then m ∠ 1 + m ∠ 1=180^(∘) by substitution. Solving the equation gives m ∠ 1 = 90^(∘). Since m ∠ 1 = m ∠ 2, then m∠ 2 is also . Therefore, ∠ 1 and ∠ 2 are right angles.

Because angles 1 and 2 have the same measure, we know that m∠ 2 is also 90^(∘).

Given m ∠ 1 = m ∠ 2, ∠ 1 and ∠ 2 are supplementary.
Prove ∠ 1 and ∠ 2 are right angles.
Proof m ∠ 1 + m ∠ 2= 180^(∘) since they are supplementary angles. Since m ∠ 1 = m ∠ 2, then m ∠ 1 + m ∠ 1=180^(∘) by substitution. Solving the equation gives m ∠ 1 = 90^(∘). Since m ∠ 1 = m ∠ 2, then m∠ 2 is also 90^(∘). Therefore, ∠ 1 and ∠ 2 are right angles.

Our paragraph proof is now complete.