1= tan45 = tan 3o = tan( 6o/2 ) = tan( o/2 + 5o/2 )=[ tan(o/2)+ tan(5o/2) ]/[ 1- tan(o/2)tan(5o/2) ]
所以 1 = [ tan(o/2)+ tan(5o/2) ]/[ 1- tan(o/2)tan(5o/2) ]
分母移到左边 1- tan(o/2)tan(5o/2) = tan(o/2)+ tan(5o/2)
原式 = 1- tan(o/2)tan(5o/2) + tan(o/2)tan(5o/2) = 1