a/a-b = c/c-d
For this step to be correct, you must assume that a,c are non-zero (if they are, the equation holds trivially, although I don't really know what we're trying to "prove" here...).
What about ?If then , and thus .
You basically said, "assume the equation holds, then it holds". This is obviously not a valid proof!Therefore, as shown above, , as does .
Not to mention that it is quite obvious at a glance that this equation will definitely not hold for any four real numbers... what if we choose