How to find the missing angles on crossing lines

Part ofShape, space and measures

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A short video showing how to find the missing angles on crossing lines when you only know one of the angles.

How to find the missing angles on crossing lines slideshow

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Click to see a step-by-step slideshow.
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WHAT YOU NEED: ruler, pencil and coloured pens.
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STEP 1: Draw two lines which cross on a piece of paper.
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STEP 2: Label the angle you know and colour in the angle arc. Colour in the opposite angle in the same colour, and the other two angles in a different colour.
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STEP 3: The given angle here in green is 80°. The angles on a straight line always add up to 180°, so the purple angle next to it must be 100°.
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STEP 4: Work out the size of the remaining angles. Opposite angles are always equal, so the remaining green and purple angles must be 80° and 100°. All angles around a point will always add up to 360°.

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