randRange( 20, 160 ) randRange( 0, 7 ) ( KNOWN_INDEX + 4 ) % 8 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?

MEASURE

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

Notice that the blue and orange angles are in the same position but at different intersections.

One way to describe the angles is to say that they are corresponding angles.

There is a proof for why corresponding angles are equal, but for now let's just take that on faith.

The orange angle measures MEASURE degrees.

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