You don't have to show that because if I was less than 2, the I(n-2) would be negative and therefore the equation won't work will just keep on going. I0 is the last value for reduction formulaes
not that much just break it into ( tan x ) ^2 ( tan x ) ^(n-2)
then use integration by parts with u= ( tanx ) ^(n-2) and dv = ( tanx)^2 , and use the identity sin^2 + cos^2 =1 to turn tan^2 into something easy to integrate
you just have to do a few examples, most of the time you can tell how to split up the integral based on what you are trying to prove
at OP when it says prove it true for n>=2, that is just kind of a technicality that they have decided to include because as someone as said I ( 1 ) would not be defined. You dont have to prove that the reduction formula is true for n>=2 , you just prove it for general values of n but they have told you that it is only true for n>=2