sin 2b 2sinasinc

∴sinB≠0，A+B+C=180° ∵a=b，则A=B ∴C=π-2B ==>sinC=sin(2B)=2sinBcosB ∵(sinB)^2=2sinAsinC ==>(sinB)^2=2sinBsin...

a^2(1-\sin^2B)+2ab \cos A\cos B +b^2(1-\sin^2A) \\\ &= a^2+b^2+2ab\cos A\cos B-a^2\sin^2B-b^2\sin^2A ...

的连线, 其解析式为y=2xπ. 我们因而有不等式,sinx>2xπ,x∈(0,π2). 在锐角三角形中就有sinA+sinB+sinC>2A+2B+...

根据正弦定理a/sinA=b/sinB=c/sinC=2RsinA=a/2RsinB=b/2RsinC=c/2Rsin^2 A=sin^2B +sin^2Ca^2/4R^2=b^2/4R^2+c^2/4R^2a^2=b^2+c^2所以三角...

（sinB）^2=2sinAsinC 由正弦定理得 b^2=2ac

sin^2B=sinAsinC.A,C中有个是直角.设A=90度.sin^2B=sinC,两边都是小于1的正数,则sinB>sinC,B>C,C最小.sin^2B=sinC=sin(90-B)=cosB cos^2B+sin^2B=cos^...

当1-sin2B=0时， tanA(1-sin2B)=1-cos2B B=π/4+kπ。因为是三角形 k=0 B=π/4 sinA=2sinBsin(A+B) 把B=π/4带入 sinA...

原式 sinA sinB sinC / (sinA + sinB + 2sinC)利用 sinC = sin(π - A - B) = sin(A + B) = (...

利用正弦定理化简sin2A=sin2B+sin2C得:a2=b2+c2,∴△ABC为直角三角形,∵sinA=sin(B+C)=sinBcosC+cosBsinC=2sinBcosC,∴sinCcosB-cosCsinB=sin(C-B)=0,∵C-B=0,即B=...