RtADB:AB^2 = AD^2 + BD^2
RtADC:AC^2 = AD^2 + DC^2
AB^2 + AC^2 = 2*AD^2 + BD^2 + DC^2
又,AD^2 = BD*DC
所以 AB^2 + AC^2 = BD^2 + DC^2 + 2*BD*DC = (BD + DC)^2 = BC^2
勾股定理知,三角形ABC为直角三角形