(i) y-z
(ii) \(\frac{x+y}{2}\)
(iii) z2
(iv) \(\frac{pq}{4}\)
(v) x2+y2
(vi) 3mn+5
(vii) 10-yz
(viii) ab-(a+b)
If \( x, y \) are two positive integers such that \( x + y = 20 \) and the maximum value of \( x^3 y \) is \( k \) at \( x = a, y = \beta \), then \( \frac{k}{\alpha^2 \beta^2} = ? \)