[see also: contrary, otherwise
To obtain a contradiction, we suppose that......
Suppose, to derive a contradiction, that......
Striving for a contradiction, suppose that......
Suppose, towards a contradiction <for contradiction>, that......
......, which is a contradiction.
Now we have the required contradiction since......
This leads to the contradiction that 0<a<b=0.
......, in contradiction with Lemma 2.
This is a clear contradiction of the fact that......