方程(15分)

ac + bd = 0
ac = -bd
a/b = - d/c

假設 a/b = - d/c = k , k 是一常數
則 a = kb 及 d = -ck

代入 a² + b² = c² + d² = 1, 得 (kb)² + b² = c² + (-ck)²
(k² + 1) b² = c² (k² + 1)
b² = c²

如b = c , 則 a = -d, R = ab + cd = (-c)d + cd = 0
如b = -c , 則 a = d, R = ab + cd = d(-c) + cd = 0

結論 : R = 0

R = 0

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