I am facing problems in recognizing the Question in Calculus as given in description ?

I have problem as I am not able to recognize the pattern in Limits that when I have to calculate both left hand and right hand limit and in which cases they are not necessary.
Because in my book some solutions to the problems are given directly placing value without calculating LHL and RHL whereas in other questions they are calculating both LHL as well as RHL even if it is not necessary?