cosA 2+cosB=3cosC 2,求sinC最小值

2a+b=(a+b)+ab=(a+b)-a为了简单设x=a+b3cos(2a+b)+5cosb=3cosxcosa-3sinxsina+5cosxcosa+5sinxsina=8cosxcosa+2s

1.Acos(a+b)cos(a-b)=(cosa*cosb-sina*sinb)*(cosa*cosb+sina*sinb)=cosa*cosa*cosb*cosb-sina*sina*sinb*s

sinA=√(1-cos²A)=√[1-(3/5)²]=4/5.sinB=√(1-cos²B)=√[1-(5/13)²]=12/13.在△ABC中,C=180-

cosa+cosb=1/3,sina+sinb=1/2分别两边平方(cosa)^2+2cosacosb+(cosb)^2=1/9(sina)^2+2sinasinb+(sinb)^2=1/4相加且(c

（cosa+cosb）（cosa-cosb）=cos^2(a)-cos^2(b)=[cos(2a)+1/]2-[cos(2b)+1]/2=1/2*[cos(2a)-sin(2a)].cos（a+b）c

cosa+cosb=1/2,sina+sinb=1/3两边平方cos²a+2cosacoab+cos²b=1/4sin²a+2sinasinb+sin²b=1/

∵sinA+cosB=-1/3∴cosB=-1/3-sinA∵-1≤cosB≤1∴-1≤-1/3-sinA≤1∴-2/3≤-sinA≤4/3∴-4/3≤sinA≤2/3∵-1≤sinA≤1∴-1≤si

(sina+sinb)^2+(cosa+cosb)^2=(sina)^2+(sinb)^2+2(sina)(sinb)+(cosa)^2+(cosb)^2+2(cosa)(cosb)=2+2[(sin

应用公式:cos(a)cos(b)+sin(a)sin(b)=cos(a-b)进行对条件的化简,得:cos(a+b-b)=cos(a)=1/3cos(2a+π/4)=cos(2a)cos(π/4)-s

3cos(2a+b)+5cosb=3cos[(a+b)+a]+5cos[(a+b)-a]=8cos(a+b)cosa+2sin(a+b)sina=0tana=-4cot(a+b)tan(a+b)tan

应该是cosA+cosB=2cos[(A+B)/2]cos[(A-B)/2]吧.

cosa+cosb=3/5.(1)sina+sinb=4/5.(2)(1)^2+(2)^2得(cosa)^2+(cosb)^2+2cosacosb+(sina)^2+(sinb)^2+2sinasin

题目是不是错了,或还差条件,如果是求(cosa)^2+(cosb)^2,结果如下:cos(a+b)cos(a-b)=(cosacosb)^2-(sinasinb)^2=(cosacosb)^2-[1-

1、(A)cosA+cosB=1/2----------------(1)sinA+sinB=1/3------------------(2)(1)²+(2)²得：2+2cos(A

cos(A-C)+cosB=cos(A-C)-cos(A+C)=cosAcosC+sinAsinC-cosAcosC+sinAsinC=2sinAsinC=3/2即sinAsinC=3/4根据正弦定理

原式=sin²a+2sinasinb+sin²b+cos²a+2cosacosb+cos²b=(sin²a+cos²a)+2(cosacos

cos[(7/5)b]和cosb之间没有倍数关系啊

cosa+cosb=3/5,平方得：cos²a+cos²b+2cosacosb=9/25①sina+sinb=4/5平方得：sin²a+sin²b+2sinas

BC是三角形内角则sin为正sin²B+cos²B=1所以sinB=4/5同理sinC=12/13sinA=sin[180-(B+C)]=sin(B+C)=sinBcosC+cos