Question #18dda

1 Answer
Ratnaker Mehta
Nov 28, 2017

# -sqrt3/2.#

Explanation:

Note that, #cos(-theta)=costheta.#

#:. cos(-870^@)=cos870^@,#

#=cos(900^@-30^@),#

#=cos(5xx180^@-30^@),#

From this, we see that #870^@# lies in the #2^(nd)# Quadrant, in

which #cos# is #-ve.#

Also, #cos(5xx180^@-theta)=-costheta.#

#:. cos(-870^@)=-cos30^@=-sqrt3/2.#

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