1.4Auxiliary Elements of a Triangle
Four ways to draw a line from a corner, and each set of three meets at one point. Find the four centres and learn what each one is for.
- I can name the four auxiliary elements of a triangle.
- I can use the angle bisector as the set of points at equal distance from two arms.
- I can use the interior and the exterior angle bisector theorems.
- I can find the angle between two bisectors.
- I can use the centroid and the two to one ratio on a median.
- I can say where the circumcentre and the orthocentre lie in any triangle.
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1.5The Area of a Triangle
One triangle has three different bases, and every one of them gives the same area. Use that to split areas, compare them and scale them.
- I can find an area from any of the three base and height pairs.
- I can compare two areas that have the same height.
- I can split an area with a line drawn from a corner.
- I can show that sliding the top corner along a parallel line does not change the area.
- I can use the medians to cut a triangle into equal parts.
- I can scale an area by the square of the similarity ratio.
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Planned
1.6The Sine and Cosine Theorems
Two sides and the angle between them, and no height anywhere. One dropped altitude gives the area, the law of sines and the cosine theorem.
- I can find an area from two sides and the angle between them.
- I can compare two areas in the case where the sine cancels.
- I can build the law of sines from the three area expressions.
- I can prove the cosine theorem when the angle is acute and when it is obtuse.
- I can use the sign of the cosine to say what kind of angle it is.
- I can find the length of a median.
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Planned