2^x×3^(z-1) = 72 = 2^3×3^2
∴ x = z = 3
代入原式得
9y²M = [1/3×3^(2n+2)×y^(n+3)×3^4]/[5×3^(2n-1)y^(n+1)×3]
= 3^5/5×y²
= 243/5y²
∴ M = 243/(5×9) = 27/5