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A pair of parallel lines is intersected by a transversal such that ∠1 and ∠2 form a pair of interior angles on the same side of the transversal. If m\angle 1=125°, what is the measure of ∠2?
45°
55°
145°
115°
55°
For parallel lines, consecutive interior angles are supplementary (add up to 180°); simply subtract the given angle from 180°.
Two parallel lines intersected by a transversal, angle 1 = 125 degrees, angle 1 and angle 2 are consecutive interior angles.
∠1+∠2=180°
For parallel lines, consecutive interior angles are supplementary (add up to 180°); simply subtract the given angle from 180°.
Confusing 'consecutive interior angles' with 'alternate interior angles' (which are equal), or forgetting that the sum is 180°.
Identify the Property
When two parallel lines are cut by a transversal, interior angles on the same side of the transversal are supplementary.
∠1+∠2=180°
Substitute Known Values
Given m∠1=125°, we substitute this into the supplementary angle equation.
125°+∠2=180°
Calculate the Result
Solve for ∠2 by subtracting 125° from 180°.
∠2=180°−125°=55°
B is correct because the sum of interior angles on the same side of a transversal intersecting parallel lines is always 180°, and 180° - 125° = 55°.
This property of interior angles is fundamental to solving problems in coordinate geometry, specifically when finding slopes or angles of lines in a plane.