In right angled triangle ACD
AC2=CD2+AD2
⇒AC2=22+42=20
In right angled triangle ADB
AB2=AD2+DB2
⇒ AB2=42+82=80
Now BC=BD+DC=8+2=10 cm
We can observe that BC2=102=100
and AB2+AC2=80+20=100
Hence BC2=AC2+AB2
Hence, from converse of Pythagoras theoren we have
∠BAC=90∘