cosa+cosb-cos(a+b)=3/2cosa+cosb=2cos[(a+b)/2]cos[(a-b)/2]cos(a+b)=2[cos(a+b)/2]^2-12cos[(a+b)/2][cos(a-b)/2-cos(a+b)/2]=1/2a=b时,2cosa(1-cosa)=1/22cosa-1=0cosa=1/2a=60