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After explaining some basic trigonometry to my kid, such as $\sin (\alpha+\beta) = \sin \alpha \cos \beta + \cos \alpha \sin \beta$, Law of sines, Law of cosines, I wonder if there are some interesting quizs for him to work on? Even better if it's a book.

Example for "interesting", $\sin 0 = \frac{\sqrt{0}}{2}$, $\sin \frac\pi 6 = \frac{\sqrt{1}}{2}$, $\sin \frac\pi 4 = \frac{\sqrt{2}}{2}$, $\sin \frac \pi 3 = \frac{\sqrt{3}}{2}$, while $\sin \frac\pi 2 = \frac{\sqrt{4}}{2}$. Or more details in wiki page Trigonometric constants expressed in real radicals.

Explanation in searching of interesting problems -- I believe exercises are necessary for one to really get the ideas and tricks of trigonometry, but they might also be boring, so if there are a bunch of interesting problems, that'll be perfect.

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There are many interesting trigonometry problems in "Trigonometry for the Practical Man" by James Edgar Thompson. Here are some from Chapter 5:

Problem 1

From the top of a mountain three miles above sea level, the angle of depression of the ocean horizon is found to be 2° 13' 50". Find the radius of the earth.

Answer: 3960 miles

Problem 2

An observer sights a telescope on the sun, then turns the telescope straight up to the zenith, measuring an angle of 30° 0' 13.2". At this same moment (by previous agreement), another observer 8280 miles away reports the sun is just setting. Find the distance from the center of the earth to the center of the sun. (You will need the earth’s radius found in problem 1.)

Answer: 92,800,000 miles

Problem 3

From a point on the surface of the earth, the sun is sighted with a transit, first on one edge and then on the other, and the angle between the lines of sight is found to be 32' 4". Find the radius of the sun. (You will need the answers to problems 1 and 2.)

Answer: 433,000 miles

Problem 4

On January 1, an astronomer measures the sun-earth-star angle of the star Proxima Centauri, and finds it to be 95° 27' 45", and six months later measures the angle and finds it has changed to 84° 32' 13.6". Find the distance from the earth to the star on January 1. (You will need to use the earth-sun distance found in problem 2.)

Answer: 4.63 light years

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  • $\begingroup$ indeed these sound quite interesting! $\endgroup$ – athos Nov 25 at 8:22
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When introducing the sum of angle equations, I practiced presenting this. Starting with introducing a right triangle inside a rectangle. (I misplaced my notes, in which I had every step clearly laid out.) In the end, it was a great proof of both equations.

enter image description here

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    $\begingroup$ indeed this is a smart construction, i'll show it to my kid, thank you! $\endgroup$ – athos Nov 22 at 17:59
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I remember enjoying simplifying trig expressions. You can find exercises all over the web. One source: math-exercises.com. Here's a snippet:


Trig


Two more sources:
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  • $\begingroup$ For example, from the snippet: (a) $1+\cos x$, (b) $\cot^2 x$, (c) $-2 \cos x \sin x$. $\endgroup$ – Joseph O'Rourke Nov 20 at 0:26
  • $\begingroup$ @joseph-orourke thank you very much, indeed these are quite interesting! $\endgroup$ – athos Nov 20 at 10:29

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