Answer
$A\bigtriangleup A^c = (A - A^c) \cup (A^c - A) \\
=(A \cap (A^c)^c) \cup (A^c \cap A^c) \\
(set\,\,dif\! ference\,\,law )\\
=(A\cap A)\cup (A^c \cap A^c)\\
(double\,\,complement\,\,law)\\
=A\cup A^c\\
by(idempotent\,\,law)\\
=U \\
(complement\,\,law\,\,for\,\cup )
$
Work Step by Step
$A\bigtriangleup A^c = (A - A^c) \cup (A^c - A) \\
=(A \cap (A^c)^c) \cup (A^c \cap A^c) \\
(set\,\,dif\! ference\,\,law )\\
=(A\cap A)\cup (A^c \cap A^c)\\
(double\,\,complement\,\,law)\\
=A\cup A^c\\
by(idempotent\,\,law)\\
=U \\
(complement\,\,law\,\,for\,\cup )
$