求sin10°sin30°sin50°sin70°的值.
证明:cos(A-B)=cosAcosB+sinAsinB.
试证(cosA+sinA)/(cosA-sinA)=tan2A+sec2A.
试证 (tana+tanb)/(tana-tanb)=sin(a+b)/sin(a-b).
设A,B,C 为一三角形之三角,试证 sin²A+sin²B+sin²C = 2+2cosAcosBcosC.
求证 sin(x+y)/sin(x-y)=(tanx+tany)/(tanx-tany)
证明 (sin²A-sin²B)/(sinAcosA-sinBcosB)=tan(A+B).