Question Video: Using the Sine Rule to Find the Length of A Side of A Triangle in A Composite Figure | Nagwa Question Video: Using the Sine Rule to Find the Length of A Side of A Triangle in A Composite Figure | Nagwa

# Question Video: Using the Sine Rule to Find the Length of A Side of A Triangle in A Composite Figure Mathematics • Second Year of Secondary School

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In the given figure, π΅πΆπ·π is a rectangle and π΅ is a point on the straight line π΄πΆ. π΅πΆ = 405 m, πβ π·π΄πΆ = 21Β°, and πβ ππ΄πΆ = 59Β°. Find the length of π·πΆ giving the answer to the nearest meter.

04:10

### Video Transcript

In the given figure, π΅πΆπ·π is a rectangle and π΅ is a point on the straight line π΄πΆ. π΅πΆ is equal to 405 meters, the measure of angle π·π΄πΆ is 21 degrees, and the measure of angle ππ΄πΆ is 59 degrees. Find the length of line segment π·πΆ, giving the answer to the nearest meter.

We will begin by adding the information from the question onto our diagram. As π΅πΆπ·π is a rectangle, all the angles inside it will be equal to 90 degrees. We are told that π΅ is a point on the straight line π΄πΆ. Therefore, the angle ππ΅π΄ is also 90 degrees. The measure of angle π·π΄πΆ is 21 degrees. And since the measure of angle ππ΄πΆ is 59 degrees, then the measure of angle ππ΄π· is 59 degrees minus 21 degrees. This is equal to 38 degrees. Finally, we are told that π΅πΆ has length 405 meters, which means that ππ· is also equal to 405 meters.

Since angles in a triangle sum to 180 degrees, the measure of angle π΄ππ΅ is 31 degrees. We are asked to find the length of line segment π·πΆ, which we have labeled π₯. There are many ways to do this. One way would be to split our shape into two triangles, π΄ππ· and π΄πΆπ·. We know that one way to calculate missing lengths in triangles is using the sine rule or law of sines. This states that π over sin π΄ is equal to π over sin π΅, which is equal to π over sin πΆ. Applying this rule to triangle π΄ππ·, we have side length π΄π· divided by sin of 121 degrees is equal to 405 divided by sin of 38 degrees. Multiplying through by the sin of 121 degrees, we can calculate the length of side π΄π·. This is equal to 563.8695 and so on. As this is not the final answer, we will not round. But we know that the length of π΄π· is 563.8695 and so on meters.

We can now consider triangle π΄π·πΆ. We could use the sine rule once again as we know the measure of two angles together with the length of one side. However, since this is a right triangle, we can simply use the sine ratio, which states that the sin of angle π is equal to the opposite over the hypotenuse. The sin of 21 degrees is equal to π₯ over 563.8695 and so on. Multiplying through by the denominator, we have π₯ is equal to 202.0727 and so on. We are asked to give our answer to the nearest meter. Since the first digit after the decimal point is zero, we round down. The length of π·πΆ correct to the nearest meter is 202 meters.

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