更新1:
我諗樓下既解法有d問題 ====================================== eg. show f(x)=x^2 is injective let f(a)=f(b) a^2=b^2 a=b f is injective let a =1 , b=-1 a=/=b but f(a)=f(b) there's a contradiction.............. ========================================
更新2:
the correct one is : a^2-b^2=0 a+b=0 or a-b =0 a=-b or a=b therefore , f is not injective..........
