由题意可求得A=36,B=C=72
1-cosA+cosB-cosAcosB
=(1-cosA)(1+cosB)
=2[cos(B/2)]^2 *2[sin(A/2)]^2
=(2cos36sin18)^2
而2cos36sin18
=2cos36sin18cos18/cos18
=sin72/2cos18
=1/2
所以原式=1/4.
由题意可求得A=36,B=C=72
1-cosA+cosB-cosAcosB
=(1-cosA)(1+cosB)
=2[cos(B/2)]^2 *2[sin(A/2)]^2
=(2cos36sin18)^2
而2cos36sin18
=2cos36sin18cos18/cos18
=sin72/2cos18
=1/2
所以原式=1/4.