Linear cubics

The sum \[ \frac{3}{1 \cdot 2 \cdot 3} + \frac{5}{2 \cdot 3 \cdot 4} + \frac{7}{3 \cdot 4 \cdot 5} + \cdots + \frac{81}{40 \cdot 41 \cdot 42} \] can be expressed as \( \frac{a}{b} \) where \(a\) and \(b\) are coprime positive integers. What is the value of \(a + b\)?

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