Video: Dividing a Triangle into Right Triangles in Order to Calculate an Unknown Length

In the figure ๐ด๐ถ = 3.5. What is ๐ด๐ต? Give your answer to two decimal places.

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Video Transcript

In the figure, ๐ด๐ถ is equal to 3.5. What is ๐ด๐ต? Give your answer to two decimal places.

Letโ€™s begin by adding the length ๐ด๐ถ to our diagram. Now weโ€™re trying to find the length of the side ๐ด๐ต, and there are two ways that we can do this. Notice that the triangle split into two right-angled triangles.

The first method we could use right angle trigonometry to calculate the length of the side ๐ด๐ท, before using it again to find the side ๐ด๐ต. However, if we look carefully, we can see that we have a non-right-angled triangle for which we know the length of one side and the measure of two of its angles. This means we can use the law of sines to calculate the missing length.

The law of sines requires the triangle to have pairs of sides and angles. Thatโ€™s to say, we know the length of the side ๐‘ and the measure of the angle at ๐ต and we know the measure of the angle at ๐ถ, and weโ€™re trying to find the length of the side ๐‘.

We can use either form for the law of sines. However, since weโ€™re trying to find a missing length, itโ€™s sensible to use the first version. This will minimise the amount of rearranging we need to do. Similarly, if weโ€™re trying to find a missing angle, weโ€™d use the second version of the formula.

We know neither the measure of the angle at ๐ด nor the length ๐‘Ž. So weโ€™re going to use these two parts of the formula: ๐‘ over sin ๐ต equals ๐‘ over sin ๐ถ. Substituting each of these measurements into our formula gives us 3.5 over sin of 63 equals ๐‘ over sin of 41.

To solve the equation, we can multiply both sides by sin of 41. That gives us 3.5 over sin of 63 multiplied by sin of 41. Typing that into our calculator gives us 2.5770. Correct to two decimal places, the length of ๐ด๐ต is 2.58 units.

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