Complete the proof of the exterior angle theorem
given: ∠adc is an exterior angle of ∆bcd.
prove: m∠b+m∠c=m∠adc

1. ∠adc is an exterior angle of δbcd= given
2. = triangle sum of theorem
3. ∠adc and ∠bdc form a linear pair=
4. =
5. =
6. =
7. ∠adc=m∠b+m∠c = subtraction property of equality