randRange( 20, 160 ) shuffle( randFromArray( [ [ 3, 4 ], [ 2, 5 ] ] ) ) KNOWN_INDEX % 2 === 0 ? ANCHOR : 180 - ANCHOR

The two horizontal lines are parallel, and there is a third line that intersects them as shown below.

If we know that the blue angle is MEASURE degrees, what is the measure of the orange angle?

180 - MEASURE

init({ range: [ [ -1, 11 ], [-1, 4] ] }); graph.pl = new ParallelLines( 0, 0, 10, 0, 3 ); graph.pl.draw(); graph.pl.drawTransverse( ANCHOR ); graph.pl.drawAngle( KNOWN_INDEX, true ); graph.pl.drawAngle( UNKNOWN_INDEX, false, "#FFA500" );

The pink angles are adjacent to the blue angle and form a straight line, so we know that they measure 180 - MEASURE degrees.

graph.pl.drawAdjacentAngles( KNOWN_INDEX, true, "#FF00A5" );

It makes sense that the pink angles are equal because they are opposite each other.

One of the pink angles corresponds with the orange angle, and the other pink angle forms an alternate exterior angle.

The orange angle equals the pink angles and measures 180 - MEASURE degrees.

graph.pl.drawAngle( UNKNOWN_INDEX, true, "#FFA500" );

Note that the blue and orange angles are supplementary.