The contrapositive of a conditional can be formed by interchanging the conclusion
and the hypothesis and negating both Example 6 Prove that the function f : R →
→
→
→
→ R  defined by f (x) = 2x + 5 is one-one Solution A function is one-one if f (x1) = f (x2) ⇒ x1 = x2 Using this we have to show that “2x1+ 5 = 2x2 + 5” ⇒ “x1 = x2”