Find the equation of tangent of tangent of the curve y = `b * e^(-x//a)` at that point at which the curve meets the Y-axis.

Explanation

`y=b*e^(-x//a) …(1)`
At `Y`-axis , `x=0`
`:. Y=be^(0)= b`
So , we will find the equation of tangent at point `(0, b)` . Now differentiate eq. `(1)` with respect to `x` .
`(dy)/(dx) =-b/a * e^(-x//a)`
Slope of tangent at point `(0 , b)` is
`m =-b/a * e^(0)=-b/a`
and equation of tangent
` y-b=-b/a*(x-0)`
`rArr ay - ab=-bx`
`rArr bx + ay = ab`.