20)
check the picture below, keep in mind that negative angles move about clockwise, same way a clock goes.
21)
[tex]\bf \cfrac{17}{6}\implies \cfrac{6+6+5}{6}\implies \cfrac{6}{6}+\cfrac{6}{6}+\cfrac{5}{6}\implies 2+\cfrac{5}{6}
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therefore\qquad \cfrac{17\pi }{6}\implies \boxed{2\pi +\cfrac{5\pi }{6}}\implies 2\frac{5}{6}\pi
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2\pi \textit{ is one full }\stackrel{revolution}{go~around}\textit{ and then the angle goes a bit more further}
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\cfrac{5\pi }{6}\textit{ more, which lands it back on the I quadrant}[/tex]
now, the little bit more makes that angle, if we nevermind the first revolution, one could say that angle started really from 0, though we know it didn't, however if that angle did so, is also a co-terminal angle of 17π/6.
