How do you find the derivative of #cos(-x)#?

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Answer 1 · 2015-10-22T22:56:24

See the explanation.

Before learning the chain rule, you need to use a fact from trigonometry:

#cos(-x) = cosx#

Therefore,

#d/dx(cos(-x) = d/dx(cosx) = -sinx#

(By the way #sin(-x) = -sinx#, so the answer couls be written #sin(-x)#.

Using the chain rule

#d/dx cos(-x) = -sin(-x)*d/dx(-x)#

# = -sin(-x)(-1) = sin(-x) = -sinx#