I can just plug in zero but my teacher says that we can't use the direct substitution property with trig functions even if you can get the correct answer, which is lame. So how do I solve this? And yes its sin(cos x) not sin x(cos x)
Because the teacher says the definition for direct substitutution property can only be used for polynomial and rational functions.
Theorem: (Direct Substitution Property) Suppose f(x) is a polyno-
mial or a rational function and a is in the domain of f.
http://web.viu.ca/wattsv/math121/Ove...bstitution.pdf
He said that "using the direct substitutution property for trig functions won't be theromatically (if thats a word) correct."
Seriously someone needs to revise the direct substitutution property so that it will be "theromatically" correct for trig functions.