Every point in the unit circle is identified either by its coordinates [tex](x,y)[/tex] or by the angle it forms with the x-axis, [tex]\alpha[/tex].
The trigonometric functions associate with every angle [tex]\alpha[/tex] and the correspondant [tex](x,y)[/tex] coordinates the two values
[tex]\cos(\alpha)=x,\quad\sin(\alpha)=y[/tex]
This procedure can be done for every angle [tex]\alpha[/tex], so you don't have to work with acute angles only.
