余弦定理，正弦定理，射影定理

已知:三角形中角A=90度,AD是高.(1)用勾股证射影:因为AD^2=AB^2-BD^2=AC^2-CD^2,所以2AD^2=AB^2+AC^2-BD^2-CD^2=BC^2-BD^2-CD^2=(BD+CD)^2-(BD^2+CD^2)=2BD*CD.故AD^2=BD*CD.运用此结论可得:AB^2=BD^2+AD^2=BD^2+BD*CD=BD*(BD+CD)=BD*BC,AC^2=CD^2+AD^2=CD^2+BD*CD=CD(BD+CD)=CD*CB.综上所述得到射影定理.(2)用射影证勾股:因为AB^2=BD*BC,AC^2=CD*CB,所以AB^2+AC^2=BD*BC+CD*CB=BC(BD+CD)=BC^2.