The general formula  CnH2n−2 is true for

[Step 1]:Alkyne is an open chain unsaturated hydrocarbon. It contains one triple bond  between carbon and carbon atoms. Therefore four hydrogen atoms are relatively less   than alkane.[Step 2]:Alkadiene is an open chain unsaturated hydrocarbon. It contains two double bonds.  Therefore four hydrogen atoms are relatively less than alkane. [Step 3]:Cyclo alkene is a ring chain compound having one double bond. Due to cyclic form  two hydrogen’s and presence of one double bond two hydrogens, total four  hydrogens are relatively less than  alkane.