sina 3sinc=3sinb

sina+cosb=1/3,平方sin^2a+2sinacosb+cos^2b=1/9sinb-cosa=1/2,平方sin^2b-2cosasinb+cos^2a=1/4相加2-2（sinacosb

sinA+sinC=2sin[(A+C)/2]×cos[(A-C)/2]=2sin[(π-B)/2]×cos(π/6)=2sin(π/2-B/2)×√3/2=√3cos(B/2)=√3×(1+cosB

[-1.5,0.5]

由题意得：2sinA=sinB+cosB(1)sinC^2=sinB*cosB(2)将（1）平方与（2）综合得到4sinA^2=1+2sinC再由倍角公式得到：2cos2A=cos2C将此式平方,再次

3sin^2a+2sin^b=2sina2sin^2a+2sin^b=2sina-sin^2a2*(sin^2a+sin^b)=-(sina-1)^2+1-1≤sina≤1-4≤-(sina-1)^2

打开平方得：sin^2A+sin^2B-sin^C=sinA*sinB正弦定理sinA=a/2R其它也一样a2/4R2+b2/4R2-c2/4R2=ab/4R2a2+b2-c2=ab余弦定理a2+b2

sinb=sin[(a+b)-a]=sin(a+b)cosa-cos(a+b)sina=sin(a+b)cosa-sinb/sina*sina=sin(a+b)cosa-sinb2sinb=sin(a

由√(1-2sinB),√(2sinB-1)则有:1-2sinB>=02sinB-1>=0解得:sinB=1/2则只能是sinB=1/2则此时:sinA=0+0+√3/2=√3/2由于A,B为锐角则由

2tana+3sinb=74tana+6sinb=14tana-6sinb=1相加得5tana=15tana=3,cosa=(1/3)sina因为a是锐角,所以sina>0sin²a+cos

sinA/sinB=cosB/cosA即sinAcosA=sinBcosBsin2A=sin2B2A=2B2A=180-2B.

sinC-cosC=√2sin(C-π/4)=√2cos(3π/4-C)=-√2cos(π/4+C)显然C>=π/4,否则cosC>=√2/2,sinC+sinB=√3+cosC>=2,不可能成立因此

亲,我写给你,你要给我好评哦～～～再问：恩再问：答案呢再答：等下再答：再问：亲，答案有正和负再答：那是平方，忘了～”再问：亲，那个怎么变形等于1我看不懂再答：用三角函数再问：sina的平方/4+9/4

根据A-C=π/3A+C=π-B可得A,C(包含有B的代数式）代入sinA+sinC=2sinB中可解出B

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

sinA*(sinB+cosB)-cosC=0sinA*(sinB+cosB)+cos(A+B)=0sinA*(sinB+cosB)+cosAcosB-sinAsinB=0sinAcosB+cosAc

2(sin²a+sin²b)=-sin²a+2sina=-(sina-1)²+12sin²b=2sina-3sin²a因为0

∵A+B=120º∴C=180º-(A+B)=60º∵c=3根据正弦定理2R=c/sinC=3/(√3/2)=2√3∴sinA=a/(2R)=a/(2√3)sinB=b/

∵acosA+bcosB=ccosC∴sinAcosA+sinBcosB=sinCcosC∴sin2A+sin2B=sin2C=sin(2π-2A-2B)=-sin(2A+2B)∴0=sin2A+si

显然a=b=0时,原式子成立所以cosa=1(cosa）²=1因为tana=3tanb所以sina/cosa=3sinb/cosb因为sina=2sinb所以2sinb/cosa=3sinb