This is a free lesson from our course in Trigonometry This lesson introduces trigonometric techniques for indirect measurement of tall objects i.e. you wouldn't take a ruler or a tape to measure the distance or an angle. Note that this might seem complicated here in text, but once you have instructor explain it to you in voice and handwriting in the video, you will find it much simpler. In this you'll explore how the angles and side-length properties of similar right-angled triangles may be applied in measurement situations to calculate unknown side lengths and angles. You can also use equivalent ratios to calculate the lengths of a triangle's sides: the height of an object or the length of a shadow. The next explanation focuses on the properties of similar isosceles right-angled triangles, where you use ratio in three pairs of similar isosceles right-angled triangles to find the unknown value on one triangle. (More text below video...)
Other useful lessons:
 Angles of Elevation and Depression Real-World Applications

(Continued from above) You can apply formulas to other parts of a triangle to determine or indirectly measure a distance or an angle. For example, if an airplane measures the distances to two battleships in the sea 910 m and 1000 m and the angle formed by the lines connecting the ships to the plane is 47 , to find distance between two ships, you can use the formula b2 = a2 + c2 - 2ac cos B, that yields the answer, b = 776.1 m.

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