若A+B+C =nπ (n 为整数).
求证:sin2A + sin2B + sin2C = (-1)n-1 · 4sinA · sinB · sinC
若A+B+C =nπ (n 为整数).
求证:sin2A + sin2B + sin2C = (-1)n-1 · 4sinA · sinB · sinC
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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).