△ABC中,ABC所对的边分别为a,b,c,已知a=3,cosA=√6/3,B=A+π/2,①求b的值②求△ABC的面积

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  • sinA=√3/3,sinB=sin(A+π/2)=cosA=√6/3.根据正弦定理:a/sinA=b/sinB,3/(√3/3)=b/(√6/3).所以b=3√2.C=π-A-B=π-A-A-π/2=π/2-2A.sinC=sin(π/2-2A)=cos2A=2(cosA)^2-1=1/3

    所以S=(1/2)*a*b*sinC=(1/2)*3*3√2*(1/3)=3√2/2

    由正弦定理得a/sinA=b/sinB,1/sinA=√3/sin(2A)=√3/(2sinAcosA)

    所以2cosA=√3,cosA=√3/2 .所以A=30度.所以B=60度,所以C=90度.所以c=√(a^2+b^2)=2

    由正弦定理得:b/sinB=c/sinC,2/(1/2)=c/(√2/2) ,得c=2√2

    A=π-B-C=7π/12.sinA=sin(7π/12)=sin(π/3+π/4)=(√6+√2)/4

    所以S=(1/2)*b*c*sinA=(1/2)*2*2√2*(√6+√2)/4=√3+1,即三角形面积为√3+1.