三角形内角和180°,即：A+B+C=180°
又因为：A+C=2B
所以解得：B=60°,A+C=120°,C=120°-A
cos2A=2cos²A-1,cos2C=2cos²C-1
所以：cos²A=（cos2A+1）/2
cos²C=（cos2C+1）/2
所以：cos²A+cos²C=（cos2A+1）/2+（cos2C+1）/2
=(cos2A+cos2C)/2+1
={2cos[(2A+2C)/2]cos[(2A-2C)/2]}/2+1
=cos(A+C)cos(A-C)+1
=1-[cos(A-C)]/2
=1-[cos(A-120°+A)]/2
=1-cos(2A-120°)/2
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