在△ABC中,sin^2A cosA=5 4,b c=根号3a
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AO⊥BC证明:连接AO∵AB=AC,OB=OC,OA=OA∴△ABO≌△ACO(SSS)∴∠BAO=∠CAO,∠ABO=∠ACO∴AO平分∠BAC∴AO⊥BC(三线合一)或:证明:连接AO,延长AO
sin^2A+sin^2B=sin^2C利用三角形正弦定理sinA/a=sinB/b=sinC/c显然a^2+b^2=c^2所以边c所对的角C为直角.
原式可化为a^2+b^2-c^2=ab也即是a^2+b^2-c^2/2ab=1/2也即是cosC=1/2所以C=60°联立2sinC=sinA+sinB可得等边三角形
sin²A+sin²B=2sin²C由正弦定理a^2+b^2=2c^2代入余弦定理:cosC=(a^2+b^2-c^2)/(2ab)=c^2/(2ab)>0所以:cosC
sin²A=sin²B+sin²C,a/sinA=b/sinB=c/sinC=2R(a/2R)^2=(b/2R)^2+(c/2R)^2a^2=b^2+c^2,ABC是直角
(1)∵AB=AC,∴∠ABC=∠ACB,∵OB=OC,∴∠OBC=∠OCB,∴∠ABC-∠OBC=∠ACB-∠OCB即∠ABO=∠ACO(2)方法①∵AB=AC,OB=OC,AO=AO,∴△AOB≌
AO⊥BC延长AO交BC于D∵OB=OC,AB=AC,AO=AO∴三角形ABO≌三角形ACO∠ABO=∠ACO,∠ABC=∠ACB,∠OBC=∠OCB,∴∠OBC=∠OCB,∠BOD=∠COD,OB=
作正△CAQ,连结BQ,依题意易得:∠BAQ=60°-50°=10°=∠OAB;∠QCB=80°-60°=20°;CQ=CA=CB所以∠CBQ=80°,∠ABQ=∠CBQ-∠CBA=80°-50°=3
锐角三角形,高中数学题做过.
由正弦定理和已知可以得到:a^2=b^2+c^2.所以三角形为直角三角形.
sin^2A+sin^2B=sin^2C=sin^2(A+B)=(sinAcosB+sinBcosA)^2=sin^2Acos^2B+sin^2Bcos^2A+2sinAcosAsinBcosB左边减
a²≤b²+c²-bcbc≤b²+c²-a²1/2≤(b²+c²-a²)/2bccosa≥1/2a≤60°
这是个直角三角形用正弦定理证明a/sinA=b/sinB=c/sinC=ksinA=a/k,sinB=b/k,sinC/c/k代入sin²A=sin²B+sin²C即可得
sin^2A+sin^2B+sin^2C=(1-cosA)/2+(1-cosB)/2+(1-cos^2C)=2-cos(A+B)cos(A-B)-cos^2C=2+cosCsoc(A-B)-cos^2
1、整理易得(2b-根号3.c)cosA=根号3.a.cosC,因为cosC=(a^2+b^2-c^2)/2ab得cosA=根号3(a^2+b^2-c^2)/2b(2b-根号3.c)所以角度A=arc
sin²A-sin²(180-A-B)=sinAsinB-sin²Bsin²A-sin²(A+B)=sinAsinB-sin²Bsin&su
由题意:1-sin^2A=cos^2Asin^2B+cos^2C+2sinAsinBcos(A+B)==sin^2B+cos^2C-2sinAsinBcosC=sin^2B+cosC(cosC-2si
/c=sinB/sinC&bsinB=csinC=>sinB/sinC=c/b=>b/c=c/b=>b^2=c^2i.e.b=c=>B=C=>A=180度-2B=>sinA=sin(2B)=>sin^
延长BO交AC于E,∵∠A=50°,∠ABO=20°,∴∠1=50°+20°=70°,∵∠ACO=30°,∴∠BOC=∠1+∠ACO=70°+30°=100°
改了结果相同由正弦定理a/sinA=b/sinB=c/sinC(sinA)^2=(sinB)^2+(sinC)^2等价于a^2=b^2+c^2可知△ABC直角三角形A=π/2sinA=2sinBcos