posted by jeff .
I meant to type this one:
If the sine of an acute angle is 0.8, what is the sine of twice that angle?
1.60 or .64?
Neither. Sine can never exceed 1.
If the sine is 0.8, the cosine is 0.6.
(Think of a 3,4,5 right triangle, or use cos^2 = 1 - sin^2)
The sine is 2x is 2 sinx cosx.
In this case, that is 2*(3/5)(4/5)= 24/25