※ 引述《Proakis (John G. Proakis)》之銘言：
: Prove the identity
: tan(x-y)+tan(y-z)+tan(z-x)=tan(x-y)tan(y-z)tan(z-x)
x - y + y - z + z - x = 0
A = x - y
B = y - z
C = z - x
tan B + tan C
---------------
tanA + tan(B+C) tanA + 1 - tan B tan C
0 = tan(A+B+C) = ---------------------- = --------------------------
1 - tan A tan(B+C) 分母不重要
tan A - tan A tan B tan C + tan B + tan C
= ------------------------------------------
分母不重要
把分母不重要乘到對岸, 代回 A B C , 就證完.
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