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Sine is = to opposite side/hypotenuse,

using a triangle with sides of opposite ~ 1, adjacent ~ 1 and hypotenuse ~ root(2),

hypotenuse is always positive,

hypotenuse = root(adjacent.squared * opposite.squared)

if either is negative, squaring them brings it back to a positive

and as the opposite side is the rise from the base line ( 0 degrees through to 180 degrees) it is positive in both quadrants 1 and 2

in quadrant 1, sine= 1/root(2) which is positive

in quadrant 2, sine= 1/root(2) which is positive

in quadrant 3, sine= -1/root(2) which is negative

in quadrant 4, sine= -1/root(2) which is negative

so sine is positive in quadrants 1 and 2 and is negative in 3 and 4

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Q: Why is the sine positive in quadrants 1 and 2?
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