Exercise 9 Page 428

Two angles are supplementary if the sum of their measures is 180^(∘). Since we are told that ∠5 is a supplement of ∠6, we can create an equation to demonstrate this relationship. m∠ 5 + m∠ 6= 180^(∘) We can find m∠ 6 by substituting the given value, m∠5=78^(∘), into the equation.

m∠ 5 + m∠ 6= 180^(∘)

Substitute

m∠ 5= 78^(∘)

78^(∘) + m∠6= 180^(∘)

SubEqn

LHS-78^(∘)=RHS-78^(∘)

m∠ 6= 102^(∘)

The measure of ∠ 6 is 102^(∘).