L025 · Allow 10–15 minutes for learners ready to move on. Answers are available after an attempt.
Two parallel lines have co-interior angles 3x+10 and 2x+20. Find x.
5x+30=180, so x=30°.
A student says alternate angles always add to 180°. Correct the claim.
Alternate angles on parallel lines are equal; co-interior angles add to 180°.
Lesson L025: Angle facts and parallel lines
Objective: Calculate missing angles and justify each step using named angle facts and marked parallel lines.
60-minute structure: 0-7 Connect & Check; 7-14 Share; 14-25 Present/model; 25-44 Apply route choice/practice; 44-52 independent exam-style check; 52-60 Recall & Review, WWW/EBI and next step.
I selected the correct geometric fact, theorem, formula or transformation and used a clear diagram with appropriate labels and units.
I will name the fact or theorem, show the substitution or construction step and check whether the size, direction and units of the result are reasonable.
Stretch: Two parallel lines have co-interior angles 3x+10 and 2x+20. Find x.
Deep challenge: A student says alternate angles always add to 180°. Correct the claim.