cos(a-b)=-(4/5)cos(a+b)=12/13a-b=(π/2、π) (a+b)=(3π/2 2π) 求：c

已知三角型abc三内角a b c 成等差数列,且a-c=π/3,求cos^2a+cos^2b+cos^c的值 已知cos(a-b)=-12/13 cos(a+b)=12/13且π/2 已知sin（a+2b）=4/5,cos(a+b)=12/13(a+2b,a+b）属于（0~2π）求sinb a,b为锐角,cos(a+b)=12/13 cos(2a+b)=4/5 求cosa 在△ABC中,已知cos²(π/2-A)+cosA=5/4,且b+c=根号3a 求cos(B-C)/2 已知0<A<π/2<B<π,cos(B-π/4)=1/3,sin(A+B)=4/5求cos(a+ 已知sin(a+b)=-3/5,cos(a-b)=12/13,且π/2 已知cos(a+b)=0,求值:sin(π+2a+b)-cos(-π/2+2a+3b) Cos(a+b)*cos(a-b)=1/5 求cos ^2-sin^2 已知COS(A+B)=12/13,COS(2A+B)=3/5,若A∈(0,π/2),求COSA的值 已知π/2＜B＜A＜3π/4,cos(A-B)=12/13,sin(A+B)=-3/5,求sin2A 求证:a^2(cos^2b-cos^2c)+b^2(cos^c-cos^2a)+c^2(cos^2a-cos^2b)=0