$\frac{1}{x} + \frac{1}{y} = \frac{1}{2}$
$\frac{x+y}{xy} = \frac{1}{2}$ ---(1)
$\frac{1}{x} + \frac{1}{y} = \frac{1}{2}$
$\frac{1}{x^2} + \frac{2}{xy} + \frac{1}{y^2} = \frac{1}{4}$
$\frac{2}{xy} = \frac{1}{4} - \frac{5}{36} = \frac{1}{9}$
$xy = 18$
แทนค่าใน (1)
$x+y = 9$
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