三角形内角ABC所对边abc.a＝1b＝2cosC＝1/4
a=1b=2
cosC=(a^2+b^2-c^2)/2ab=1/4
2(1+4-c^2)=2
c=√5
周长=a+b+c=3+√5
∵C<180
∴sinC>0
∴sinC=√(1-cos^2C)=√15/4
a/sinA=c/sinC
sinA=asinC/c
=√15/4
/√5
=√3/4
∵a
0
cosA=√(1-sin^2A)=√13/4
cos(A-C)=cosAcosC+sinAsinC
=√13/4*1/4+√3/4*√15/4
=(√13+3√5)/16
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