If A, B, C are three sets such that A⊂B, then prove that C−B⊂C−A
If (a + b +c + d) (a -b- c+ d)
=(a + b- c- d) (a -b+c- d);
prove that : a : b =c : d.
If a, b, c are rational numbers such that a2 + b2 + c2 − ab − bc − ca = 0, prove that a = b = c.