here the domain of function is [-1,1] as sin inverse and cos inverse are only defined for theses value of x only....
and for ever |x|<=1,
thrfore f(x) reduces to
f(x)=pi/2+tan-1x
now as tan inverse is an increasing function in given domain, so for maximum value of f(x),
x=1
=> f(x)= pi/2+pi/4
=3pi/4
and for minimum value
x= -1
=> f(x)=pi/2-pi/4
=pi/4
so shakir is absolutely correct .......