延长ad交bc于点f.
在△acd和△fcd中,∠adc = 90°= ∠fdc ,cd为公共边,∠acd = ∠fcd ,
所以,△acd ≌ △fcd ,可得：ac = fc ,ad = fd .
de是△abf的中位线,可得：de = (1/2)bf = (1/2)(bc-fc) = (1/2)(bc-ac) .