f(a+b) = f(a)f(b)put a=b=0f(0) = f(0)f(0)=> f(0) = 1case 1: for x 1 >0 (true)case 2: x=0f(0) =1 >0 (true)case 3: for x>0-xf(x) > 0 ( f(-x) >0 , f(0) =1 )