tan(90-A/2)
=tan(B/2+C/2)
=[tan(B/2) + tan(C/2)]/[1 - tan(B/2)tan(C/2)]
所以
1 - tan(B/2)tan(C/2)
= [tan(B/2) + tan(C/2)]/tan(90-A/2)
= [tan(B/2) + tan(C/2)]tan(A/2)
所以
tan(A/2)tan(B/2) + tan(B/2)tan(C/2) + tan(A/2)tan(C/2) = 1
tan(90-A/2)
=tan(B/2+C/2)
=[tan(B/2) + tan(C/2)]/[1 - tan(B/2)tan(C/2)]
所以
1 - tan(B/2)tan(C/2)
= [tan(B/2) + tan(C/2)]/tan(90-A/2)
= [tan(B/2) + tan(C/2)]tan(A/2)
所以
tan(A/2)tan(B/2) + tan(B/2)tan(C/2) + tan(A/2)tan(C/2) = 1