(sinA)^2+(cosA)^2=1
方便起见,令sinA=x ,cosA=y
(4-5y)/(3+5x)+(3-5x)/(4+5y)=[25-25(x^2+y^2)]/[(3+5x)(4+5y)](通分)=(25-25)/[(3+5x)(4+5y)]=0