triangle ABC is a right angled triangle right angled at C and D is the mid point of bc .
by applying pythagoras theorem in triangle ABC , we get
AB2=AC2+BC2
=AC2+(2CD)2 (SINCE D IS THE MID POINT OF BC)
=AC2+4CD2
=AC2+4(AD2_AC2)
( SINCE IN TRIANGLE ACD WE GET AD2=AC2+CD2) =AC2+4AD2_4AC^2
=4AD2_3AC2
THEREFORE , AB2=4AD2_3AC2.
Like if satisfied