Find the curve for which the intercept cut – off by a tangent on the x – axis is equal to four times the ordinate of the point of contact.
Let P(x,y) be the point of contact of tangent and curve y = f(x).
It cuts the axes at A and B so, the equation of the tangent at P(x,y)
Y – y = (X – x)
Putting X = 0
Y – y = (0 – x)
⇒ Y = y – x
So, A(0, y – x)
Now, putting Y = 0
0 – y = (X – x)
⇒ X = x – y
So, B(x – y,0)
Given, intercept on x – axis = 4× ordinate
⇒ x – y = 4y
⇒ y + 4y = x
⇒ + 4 =
⇒ = – 4
We can see that it is a linear differential equation.
Comparing it with
P = , Q = – 4
I.F = e∫Pdy
= edy
= e – logy
=
Solution of the given equation is given by
x × I.F = ∫Q × I.F dy + logc
⇒ x × () = ∫ – 4 ×
dy + logc
⇒ = – 4 log y + log c
⇒ = log y – 4 + logc
⇒ = log c y – 4
⇒ = c y – 4