三角形面积题目

显示全部楼层 · 2011-2-2 11:25:26

s=c^2-(a-b)^2=c^2-(a^2+b^2)+2abs=1/2absinCc^2-(a-b)^2=c^2-(a^2+b^2)+2ab=1/2absinC 即 c^2=a^2+b^2-2ab-1/2absinC
又c^2=a^2+b^2-2*a*b*CosC (余弦定理)所以 4cosC=sinc-4又 sinC^2+cosC^2=1求出cosC=-1（舍）或-15/17 sinC=8/17 tanC=-8/15 没选项啊哥哥过程肯定是没错...

千问 · 2011-2-2 11:25:26

S=1/2 (absinC)，sinC=2S/ab=2[c2-(a-b)2]/ab；c2=a2+b2-2abcosC，cosC=(a2+b2-c2)/2ab；tanC=sinC/cosC={2[c2-(a-b)2]/ab}/[(a2+b&su...

千问 · 2011-2-2 11:25:26

c^2=a^2+b^2-2abcoscs=c^2-(a-b)^2,=2ab（1-cosc）=1//2absinc16(1-2cosc+cosc方)=1-cosc方cosc=1或cosc=15/17...