# Can the sum of criss-crossing angles for two parallel lines and a secant be equal to 180 °?

July 20, 2021 | education

| In order to solve the problem, the following theorem is needed: If two parallel lines are intersected by a secant, the angles lying crosswise are equal.

Suppose the sum of the cross-lying angles can be equal to 180º. This will be true only in one case: if the cross-lying angles are equal to 90 °, i.e. the secant will be perpendicular to the two existing parallel lines.

The answer is yes, it can.

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