Finding a missing angle (SOHCAHTOA)
Once you understand the basics of trigonometry, you can use these ideas to find the size of sides in a rightangled triangle.
You need to make sure you learn the three trigonometric ratios for sine, cosine and tangent (sin, cos and tan) by heart. You will need a decent scientific calculator with the three trigonometric functions (sin, cos and tan). You also need to know how to use the "Shift" button on your calculator to get the inverses of these. To find a missing angle, follow these steps:

First Steps 
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Next Steps 
External Links
 Mymaths: Online lesson looking at applying trigonometry to reallife problems calculating angles of elevation and depression. You can also try the online homework problems.
 Manga High: Online activity with questions and solutions. Repeat several times and try to beat your score!
 IXL: Online problems with full solutions. Practise finding a missing angle in right angled triangles (beware  some problems contain surds).