prove that tan -1(1) + tan-1(2) + tan-1(3) = pie.
=tan-11+(tan-12+tan-13)
=^/4 +(^/2-cot-12 ) + (^/2-cot-13)
=^/4+^ - tan-11/2+tan-11/3
=^/4 - ^ - tan-1{ (1/2+1/3)/(1-1/2*1/3) }
=^+ ^/4 - tan-11
=^
prove that tan -1(1) + tan-1(2) + tan-1(3) = pie.
=tan-11+(tan-12+tan-13)
=^/4 +(^/2-cot-12 ) + (^/2-cot-13)
=^/4+^ - tan-11/2+tan-11/3
=^/4 - ^ - tan-1{ (1/2+1/3)/(1-1/2*1/3) }
=^+ ^/4 - tan-11
=^