Solution
Given, ABCD is a rectangle AC is diagonal
Then AD=BC and AB=CD
And ∠ABC=∠BCA=∠CDA=∠DAB=90
0
In ΔABC and ΔADC
∠ABC=∠ADC (Angle of rectangle)
AB=DC (Opposite side of rectangle )
AC=AC (Common side)
∴ΔABC≅ΔADC
∴∠ACD=∠ACB
∵∠ACD+∠ACB=90
0
..........[Angle ACB is the angle of rectangle]
∴2∠ACD=90
0
⇒∠ACD=45
0