If K is non zero constant and $k=\frac{ab}{a+b}$ then the straight line $\frac{x}{a}+\frac{y}{b}=1$passes through the point 

$$\begin{gathered} If k=\frac{ab}{a+b} \\ \Rightarrow \frac{1}{k}=\frac{a+b}{ab} \\ \Rightarrow \frac{1}{k}=\frac{1}{a}+\frac{1}{b}\Rightarrow \frac{k}{a}+\frac{k}{b}=1 \\ comparing with \frac{x}{a}+\frac{y}{b}=1 then \left(x ,y\right)=\left(k ,k\right) \\ \therefore \frac{x}{a}+\frac{y}{b}=1 passes through the point \left(k ,k\right) \end{gathered}$$