第1个回答 2016-05-15
a/b=sinA/sinB ,a/b=cosA/cosB
sinA/sinB =cosA/cosB
sinB cosA=sinAcosB
sinB cosA-sinAcosB=0
sin(B-A)=0
A=B
又,(sinA)^2(2-cosC)=(cosB)^2+1/2
(sinA)^2[1+(1+cos2A)]=1-(sinA)^2+1/2
(sinA)^2[1+2(cosA)^2)]=3/2-(sinA)^2
(sinA)^2[3-(sinA)^2]=3/2-(sinA)^2
4(sinA)^4-8(sinA)^2+3=0
[2(sinA)^2-1][2(sinA)^2-3]=0
(sinA)^2=1/2 或(sinA)^2=3/2
sinA=√2/2 或 sinA=√6/2>1(舍去)
A=π/4
A=B=π/4
C=π/2
2)A=π/4 ,a=2
2R=b/sinB=c/sinC=a/sinA=2/√2/2=2√2
S=1/2bcsinA=1/2*(2R)^2sinBsinCsinπ/4
=1/2*(2√2)^2sinBsinC*√2/2
=2√2sinBsinC
=sin(3π/4-B)
=2√2sinB(√2/2cosB+√2/2sinB)
=2sinBcosB+2(sinB)^2
=sin2B+(1-cos2B)
=sin2B-cos2B+1
=√2sin(2B-π/4)+1
B在(0,3π/4)
2B-π/4在(-π/4,5π/4)
sin(2B-π/4在(-π/4,5π/4)值域为:这(-√2/2,1]
S=√2sin(2B-π/4)+1值域为:这(0,1+√2]
S的取值范围是:(0,1+√2]