中线定理（pappus定理）是指三角形abc内bm=mc,则ab^2+ac^2=2*(am^2+bm^2)
证明：ac^2=ah^2+hc^2ab^2=ah^2+bh^2=ah^2+(hc+2mh)^2=ah^2+hc^2+4mh*hc+4mh^2左边=ab^2+ac^2=2*ah^2+2ch^2+4mh*ch+4mh^2右边=2*(am^2+bm^2)=2*(ah^2+mh^2+(ch+mh)^2)=2*(ah^2+mh^2+ch^2+2ch*mh+mh^2)得证