姚纯
幼苗
共回答了14个问题采纳率:100% 举报
2R(Sin²A-Sin²C)=(√2*a-b)*SinB (1)
a/SinA=b/sinB=c/sinC=2R
sinA=a/(2R) sinB=b/(2R) sinC=c/(2R)
(1)整理为:c²=a²+b²-√2 ab (2)
由余弦定理:c²=a²+b²- 2ab cosC (3)
(3)-(2)得:√2 ab =2ab cosC
cosC=√2 /2
sinC=√2 /2
△ABC面积=absinC/2=2R²sinAsinBsinC
=√2R²sinAsinB
=√2R²[cos(A-B)-cos(A+B)]
=√2R²{cos(A-B)-cos[180-(A+B)]}
=√2R²[cos(A-B)+cosC]
=√2R²[cos(A-B)+√2 /2]
=√2R²cos(A-B)+R²/2
≤√2R²+R²/2(A=B即等腰三角形时面积最大)
△ABC
面积的最大值为√2R²+R²/2
1年前
8