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http://oi47.tinypic.com/nv1pbs.jpg

Why is $x$ in Figure 1 $x$ and not $-x$? This has caused me to not understand why $\cos(-\theta) = x$ and $\cos(\theta) = x$.

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    because $x$ is the projection of the point $(x,y)$ over the X-axis, wich represent $\cos\theta$2012-04-25

1 Answers 1

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The point has co-ordinates $(x,y)$. In the diagram, $x$ is a negative number, but that's OK - variables are allowed to stand for negative numbers. The angle $\theta$ is in the second quadrant, where the cosine function is negative, so $\cos\theta=x$ is perfectly consistent.

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    @Henning, you might be right.2012-04-25