Why we put m sign in demonstrative geometry in theorem?
please tell me while proving theorems
in statements why there is m symbol before angles like m<ABC + m<BCD=180
thanks .
As I said above, that is a matter determined by the author of your text material. Consult your textbook.
All one can say is that in general $\displaystyle m\angle ABC$ indicates the measure of an angle.
Whereas, the notation $\displaystyle \angle ABC$ stands for the angle itself: the union of two rays $\displaystyle \overrightarrow {BA} \cup \overrightarrow {BC} $.
Ok one confusion remains ,i have noticed that m symol is everywhere in the statements , but the author put it where there is equal sign(=) like sum of angles e.g m<ABC + m<BCD=90 here is m , where there is congruency of anlges there is no 'm' sign e.g <ABC=~ <BCD (=~ is for congrueny ) :-) so now could please define y is 'm' sign here ?
To add, when we take a sum of two angles, we must use $\displaystyle m$ to convert angles as figures to numbers first and then to add the results. That's why $\displaystyle m$ is used in $\displaystyle m\angle ABC + m\angle BCD=90$. To say that two angles are congruent, we don't need $\displaystyle m$ since we could say $\displaystyle \angle ABC\cong \angle BCD$. The latter fact is equivalent to $\displaystyle m\angle ABC = m\angle BCD$.