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The sketch above shows Mary, on the right, and she must know her height, which is the length FG. Focusing our line of sight on the top of Mary's head, the line is EF. By clicking the screen where our line of sight is focused, we get two points: A and B. AB gives the angle measurement of the line of sight. Similarly, we can get the angle measurement of the line of sight to Mary's feet. Lines AB and CD intersect at point E, creating triangle-EFG. We can now compute the height of the triangle which is what we need if you know the degree of the angle and the length of the bottom edge, because of its shape. By contraries, we can compute the height of Mary if we know that distance.
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