Exercise 10 Page 428

Two angles are supplementary if the sum of their measures is 180^(∘). Since we are told that ∠7 is a supplement of ∠8, we can create an equation to demonstrate this relationship. m∠ 7 + m∠ 8= 180^(∘) We can find m∠ 8 by substituting the given value, m∠7=109^(∘), into the equation.

m∠ 7 + m∠ 8= 180^(∘)

Substitute

m∠ 7= 109^(∘)

109^(∘) + m∠8= 180^(∘)

SubEqn

LHS-109^(∘)=RHS-109^(∘)

m∠ 8= 71^(∘)

The measure of ∠ 8 is 71^(∘).