Arthur D. answered 09/01/18
Tutor
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Forty Year Educator: Classroom, Summer School, Substitute, Tutor
tan=sin/cos
tan (-20º) is 20º clockwise which puts you in the fourth quadrant where sin is negative and cos is positive, thus making tan negative
so, tan(-20º) is negative
tan(20º) is 20º counter-clockwise which puts you in the first quadrant where tan is positive because sin and cos are both positive
so, if tan(20º) is positive then -tan(20º) is negative
therefore tan(-20º)=-tan(20º)
by the way tan(20º)=0.36397023426