Co(-150degree)=positive or negative cos150degree?please give answer with explanation

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Answer 1 · 2018-08-14T15:40:45

See my lengthy validation logic, for #cos ( +- 150^o) = - sqrt 3/2#. ..

Use #0 <= sqrt ( x^2 + y^2 ), ( x, y ) = r ( cos theta , sin theta )#.

#Q_1 rArr x, y >= 0, Q_2 rArr x<= 0, y >= 0, Q_3 rArr x, y <= 0#

#and Q_4 rArr x >= 0, y <= 0#

Anticlocwise #150^o in Q_2# and clockwise #- 150^o in Q_3., and

in both #x<= 0#, and so, #x = r cos theta <=0#

# rArr cos (+- 150^o) <= 0.

So, for both, your calculator displays the same #- 0.866025403..#

that is 9-sd approximation to #- sqrt3/2#.

After reading my answer, I hope that you would invalidate negative

r, in problems.