<span>It is an equation: here is the proof : The key to this one is that sec^2(x) = 1 + tan^2(x). So the left side is </span> <span>cot(x) (1 + tan^2(x)) (1 + tan^2(x)) </span> <span>Expanding this term by term you get </span> <span>cot(x) + 2 cot(x) tan^2(x) + cot(x) tan^4(x), </span> <span>but since cot(x) tan(x) = 1, that turns into </span> <span>cot(x) + 2 tan(x) + tan^3(x), </span> <span>which is the same as the right side.</span>
An equation is a statement that is true for particular values of the variable. An identity, however, is a statement that is true for <em>any</em> values of the variable.
If we use 60° as x, the left hand side of the equation gives us -1.15. The right hand side gives us 4.04. These are not equal, so this cannot be an identity.
