Question

Given: line BC is parallel to line ED m∠ABC = 70° m∠CED = 30° Prove: m∠BEC = 40°

Answer 1

Answer:  Choice A

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Explanation:

We start with $m\angle\text{BEC}+m\angle\text{CED}=m\angle\text{BED}$ which is the line just above the blank line.

Then we replace the angle CED with 30 and angle BED with 70. This is the substitution rule in action. Think of it like a substitute teacher taking a temporary place of the actual teacher.

So after using the substitution rule, we get $m\angle\text{BEC}+30^{\circ}=70^{\circ}$ showing why choice A is the answer.