Remember that
In a rhombus the diagonals are perpendicular and each diagonal bisect each other
so
triangle AEB is a right triangle
and
AE=AC/2=40/2=20 cm
EB=BD/2=30/2=15 cm
Applying the Pythagorean Theorem
AB^2=AE^2+EB^2
AB^2=20^2+15^2
AB^2=625
AB=25 cm
REmember that
the rhombus has four equal length sides
so
DC=AB=25 cm
Perimeter is
P=4(25)=100 cm
