If the cosine of the angle is 3/4, draw a right-angled triangle with sides 3, and hypotenuse 4. This satisfies the Pythagorean Theorem. The 4 is given in the problem, is the calculated value for the side that is not given. Now the tangent would be (opposite side/adjacent side) = /3. The sine would be the opposite side divided by the hypotenuse.
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