so I got a trouble wrong on my homework because I simplified \$cos (x) imes cos (y)\$ to be \$cos (xy)\$, and also my teacher created on my paper that the can"t it is in simplified. I was wondering if anyone could give me one explanation as to why girlfriend can"t simplify those two.

It is depends what you mean by saying simplify...Sometimes it can be beneficial to use this identity:\$cos(x) cos(y) = frac12 left< cos(x+y) + cos(x-y) ight>\$Usually girlfriend can change trigonometric role multiplication to sum or backwards.

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Multiplication of any type of two attributes with various variables cannot be simplified into one function of one variable. This kind of leveling is very rare:

(1) \$f(x)f(y)=g(xy)\$

Examples of duty that fulfill (1) is \$f(x)=ax\$ or \$f(x)=ax^n\$.Other role that fulfill (1) most likely esoteric...

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