因为：bcosC+ccosB=a
b^2sin^2C+c^2sin^B=2bcCOSB^COSC
b^2(1-cos^2C)+c^2(1-cos^2B)=2bccosBcosA
(bcosC)^2+(ccosB)^2+2bccoscosC=b^2+c^2
(bcosC+ccosB)^2=b^2+c^2
a^2=b^2+c^2
三角形为直角三角形,A为直角