I really need help with this question..

Which of the following are true for all real numbers a,b>0? For those that are not always true, give specific values of a,b>0 where they fail

(a) 1/a +1/b = 1/(a+b)

(b) 1/(a) + b = (1+b)/a

(c) 1/a +1/b = (a+b)/ab