Question

在△ABC中，证明sin^2A=sin^2B+sin^2C-2sinBsinCcosA

具体过程

Answer 1

余玄定理：正玄定理：a/sinA=b/sinB=c/sinC=k
a=k*sinA
b=k*sinB
c=k*sinC
代入余玄定理得：cosA=（sin^2B+sin^2C-sin^2A）/2sinBsinCcosA
所以sin^2A=sin^2B+sin^2C-2sinBsinCcosA