In Unit circle, in second quadrant, why is X taken negative? [duplicate]

This question already has an answer here:

Cos theta = base / hypotenuse

Base and hypotenuse are lengths, so they should always be positive.

So why is cos 3π / 4 negative ?

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• Draw or get a computer or a graphics calculator to draw the graph of $y=cos x$ between 0 and $2\pi$ radians. – Martin Hansen Mar 11 at 20:59
• Always positive is from original definition, but extensions to all angles of a unit circle needs signed values for x and y. – herb steinberg Mar 11 at 21:03 • Why cosine is simply sine but offset by $$\pi/2$$ radians