prove that tan ^{-1}(1) + tan^{-1}(2) + tan^{-1}(3) = pie.

=tan^{-1}1+(tan^{-1}2+tan^{-1}3)

=^/4 +(^/2-cot^{-1}__2__ ) + (^/2-cot^{-1}3)

=^/4+^ - tan^{-1}1/2+tan^{-1}1/3

=^/4 - ^ - tan^{-1}{ (1/2+1/3)/(1-1/2*1/3) }

=^+ ^/4 - tan^{-1}1

=^