Question

How do you prove: `2sinx = cosx`?

Answer 1

This can not be proven since it is not (in general) true.

Explanation:

As an obvious counter example:
If `x=0`
then `sin(x)=0` and `cos(x)=1`
and obviously `2 * sin(x) != cos(x)`

Perhaps (?) you wanted to solve for `x` given this equation.