yhy1980
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共回答了18个问题采纳率:88.9% 举报
f(x)=sin2x+cos2x,故:f(A+π/8)=sin(2A+π/4)+cos(2A+π/4)=sqrt(2)sin(2A+π/2)
=sqrt(2)cos(2A)=sqrt(2)/3,即:cos(2A)=1/3,故:1-2sinA^2=1/3,A是锐角
故:sinA=1/sqrt(3),而:2cosA^2-1=1/3,即:cosA=sqrt(2)/sqrt(3)
a^2=3=b^2+c^2-2bccosA≥2bc-(2sqrt(2)/sqrt(3))bc,故:bc≤3sqrt(3)/(2(sqrt(3)-sqrt(2)))
△ABC的面积S=(1/2)bcsinA≤(1/2)*(1/sqrt(3)*3sqrt(3)/(2(sqrt(3)-sqrt(2)))
=3(sqrt(3)+sqrt(2))/4,等号成立的条件:b=c=3(3+sqrt(6))/2
1年前
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