第1个回答 2013-01-24
A+C=2B,B=π/3,a/sinA=b/sinB=c/sinC=k
①已知等式平方,1+sin2A=2
sin2A=1,2A=π/2,A=π/4,
b=√3,k=2
a=ksinA=√2
②S=ac√3/4≤sinAsinC
=[cos(A-c)-cos(A+C)]/2
A=C=B,Smax=√3/4
第2个回答 2013-01-24
2B=A+C,B=60,sin(A+45)=1,A=45
a/sin45=根号3/sin60, a=根号2
S=1/2acsin60<=根号3/8(a^2+c^2)=根号3/2[2-cos(A+C)cos(A-C)]
=根号3/2[2+cosBcos(A-C)]=根号3/2[2+1/2cos(A-C)]
,最大=5根号3/4
第3个回答 2013-01-24
sinA+cosA=√2sin(A+pi/4);A=pi/4,ABC等差,B=pi/3;正弦定理a=√2;(2)余弦定理得到2ac*cosB=a^2+c^2-b^2>=2ac-b^2;从而ac<=b^2/(2*(1-cosB));S=1/2*ac*sinB<=1/2sinB*b^2/(2*(1-cosB))=3*√3/4