Answer:
[tex]\sqrt{87}[/tex]
Step-by-step explanation:
Let AB=16,BC=5,CD=x,DA=18
BC and DA are perpendicular to CD
Draw a line from B parallel to CD intersecting DA at E.
As BCDE is a rectangle(all angles of quadrilateral=90)
Therefore BE=CD=x and ED=BC=5(opposite sides are equal in a rectangle)
⇒AD=AE+ED
18=AE+5
AE=13
⇒ By Pythogores theorem in right angled triangle ABE
[tex]AB^{2} =BE^{2} +EA^{2}[/tex]
[tex]16^{2} =x^{2} +13^{2} [/tex]
[tex]x^{2} =256-169[/tex]
x=[tex]\sqrt{87}[/tex]
