Show that the line `x/a+y/b=1` touches the curve `y=b e^(-x/a)` at the point where it crosses the y-axis.

Explanation

`y= be^(-x/a)`

`dy/dx = be^(-x/a) (-1/a) `

`dy/dx = -b/a e^(-x/a)`

now, `x/a + y/b = 1`

`ax + ay - ab = 0`

slope=`-b/a`

now, `-b/ae^(-x/a) = -b/a`

`e^(x/a) = 1`

...