KS3

How to bisect lines and angles

Part ofMathsConstructions

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A one-minute guide on how to bisect lines and angles.

How to bisect a line

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WHAT YOU NEED: a ruler, a compass and a pencil.
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STEP 1: Draw a straight line with a ruler.
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STEP 2: Put the pin of a compass at the end of the line you want to bisect. Set the compass to more than half the length of the line, and draw an arc crossing the line.
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STEP 3: Keep the width of the compass the same, and from the opposite end of the line draw another arc.
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STEP 4: Mark the two points where the curved arcs cross each other. Then use a ruler to draw a line joining the points.
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The line cuts the original line exactly in half, and is perpendicular to it. You have bisected a line!

How to bisect an angle

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WHAT YOU NEED: a ruler, a compass, and a pencil.
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STEP 1: Using a ruler draw two straight lines to make an angle.
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STEP 2: Put the pin of the compass at the point of the angle. Set the other end of the compass to a point most of the way up one of the lines and draw an arc across each line. Mark the points where the arc crosses each line.
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STEP 3: Place the pin of the compass on one of the marks that you've made. Draw an arc across the open end of the angle...
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...and making sure to keep the compass the same width, place the pin on the second mark to draw another arc that crosses the first. Mark the point where the arcs cross.
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STEP 4: Using a ruler, draw a line from the point of the angle to the mark where the two arcs cross.
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This line splits the angle exactly in half. You have bisected an angle!

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