Question 951059
In the 1st and 2nd quadrants, the 
sin is positive, being (+) / (+),
so the angle {{{ x }}} can be in
either the 3rd or 4th quadrant
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In the 3rd quadrant, the tan is
(-) / (-) ,which is positive, so the
angle {{{ x }}}  must be in the 
4th quadrant
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check:
tan is (-) / (+) 
sin is (-) / (+)
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{{{ tan(x) <0 }}}
{{{ sin(x) < 0 }}}
OK