question in lebesgue measure: Given that $T$ is a Jordan set of positive Lebesgue measure, $l(T)>0$. If $M \subset T $ such that $l(M)=0$ where $l(\cdot)$ denote Lebesgue measure, is it true relation that
$$ l(T)=l(T\backslash M) $$ where $T\backslash M$ is the set $T$ excluding $M$.
Yes: in fact, for any Lebesgue measurable sets $A,B$ with $B \subseteq A$, we have $$ \ell(A \setminus B) = \ell(A) - \ell(B) $$ When $B$ has measure $0$ and $A$ is a Jordan set, we get the statement you were looking for.