因为aCOS(B+C)+bCOS(A+C)=cCOS(A+B)所以aCOS(180-A)+bCOS(180-B)=cCOS(180-C)-aCOSA-bCOSB=-cCOSCaCOSA+bCOSB=cCOSC利用余弦定理a*[(b^2+c^2-a^2)/2bc]+b*[(a^2+c^2-b^2)/2ac]=c*[(a^2+b^2-c^2)/2ab]化简得a^4+b^4-2...