Subject: Double Angle Identities
Date: Fri, 18 Jun 1999 20:08:54 -0600

Name: Ryan Grade 12 student

If sinA = 4/5, and A is a first quadrant angle, find sin4A

Hi Ryan
From the right triangle in the diagram with sin A = 4/5, Pythagoras' Theorem shows that the third side of the triangle has length 3 and hence cos A = 3/5. Use the exprression

sin 2A = 2 sin A cos A

to calculate sin 2A.

Now start again. Since you know sin 2A use Pythagoras' Theorem to find cos 2A and hence calculate sin 4A.


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