To show that a function f is injective, we have to consider all x1,x2 belonging to the domain with x1 , x2 andcheckthat f(x1) , f(x2). However,toshowthatafunctiongisnotinjective,itsufficestofindtwodifferent elements x1,x2 ofthedomainsuchthatg(x1) = g(x2). Belowwegiveageometricmethodtodeterminewhether a function is injective or not