已知 $\sin\left(a + \frac{\pi}{3}\right) = \frac{3}{5}$,求 $\cos\left(\frac{\pi}{6} - a\right)$ 的值。
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### 解法一:利用互余关系
观察角度 $\left(a + \frac{\pi}{3}\right)$ 和 $\left(\frac{\pi}{6} - a\right)$ 的关系:
$$
\left(a + \frac{\pi}{3}\right) + \left(\frac{\pi}{6} - a\right) = \frac{\pi}{2}.
$$
根据三角函数的互余关系 $\sin\theta = \cos\left(\frac{\pi}{2} - \theta\right)$,可得:
$$
\cos\left(\frac{\pi}{6} - a\right) = \sin\left(a + \frac{\pi}{3}\right) = \frac{3}{5}.
$$
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### 解法二:角度和差公式展开
1. **已知条件展开**:
$$
\sin\left(a + \frac{\pi}{3}\right) = \sin a \cos\frac{\pi}{3} + \cos a \sin\frac{\pi}{3} = \frac{1}{2}\sin a + \frac{\sqrt{3}}{2}\cos a = \frac{3}{5}.
$$
2. **目标表达式展开**:
$$
\cos\left(\frac{\pi}{6} - a\right) = \cos\frac{\pi}{6}\cos a + \sin\frac{\pi}{6}\sin a = \frac{\sqrt{3}}{2}\cos a + \frac{1}{2}\sin a.
$$
对比两式可得:
$$
\cos\left(\frac{\pi}{6} - a\right) = \frac{3}{5}.
$$
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### 最终答案
$$
\boxed{\dfrac{3}{5}}
$$