If a, b, c are in AP, prove that (a−c)2=4(b2−ac).
Since a, b, c are in AP, we have b=12(a+c)
∴ RHS=4(b2−ac)=4×{14(a+c)2−ac}
=(a+c)2−4ac=(a−c)2=LHS
Hence, (a−c)2=4(b2−ac)