90 degree anticlockwise.
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It is an anticlockwise rotation through 90 degrees.
180 degrees in the plane perpendicular to the xy plane. In general, no rotation in the (x, y) plane will take it to (-x, y) unless x = y (or -y) and, in that case it is a 270 degree clockwise rotation.
(x; y) --> (x.cos45 + y.sin45; x.sin45 - y.cos45)
Short answer: 72 degrees Longer answer: To rotate a star until it looks the same you need to make 1/5 of a complete 360 degree turn (since a star has 5 points). Sice 1/5 X 360 = 72, the answer is 72 degree angle rotation.
Suppose you wish to differentiate x/f(x) where f(x) is a differentiable function of x, and writing f for f(x) and f'(x) for the derivative of f(x), d/dx (x/f) = [f - x*f']/(f2)