cosa+cosb-cos(a+b)=3/2
--->2cos(a/2+b/2)cos(a/2-b/2)-{2[cos(a/2+b/2)]^2-1}=3/2
--->[cos(a/2+b/2)]^2-cos(a/2-b/2)cos(a/2+b/2)+1/4=0
--->[cos(a/2+b/2)-1/2*cos(a/2-b/2)]^2+{1-[cos(a/2-b/2)]^2}/4=0
--->.................................+1/4*[sin(a/2-b/2)]^2=0
--->cos(a/2+b/2)-1/2*cos(a/2-b/2)=0....(1);
并且1/2*sin(a/2-b/2)=0...................(2).
由(2)及0<a/2,b/2<π/2--->a=b
代入(1):cosa-1/2*cos0=0--->cosa=1/2
0<a<π--->a=π/3;b=π/3.
