What is the coordinate of a point $P$ on the line $2x-y+5=0$ such that $|PA-PB|$ is maximum where $A=(4,-2)$ and $B=(2,-4)$ What is the coordinate of a point $P$ on the line $2x-y+5=0$ such that $|PA-PB|$ is maximum where $A=(4,-2)$ and $B=(2,-4)$.

Let the coordinates of the point $P$ be $(x_1,y_1).$
$P$ lies on the line $2x-y+5=0$,so $2x_1-y_1+5=0$
$|\sqrt{(x_1-4)^2+(y_1+2)^2}-\sqrt{(x_1-2)^2+(y_1+4)^2}|$ is maximum.
But i dont know,how to solve it further and find $(x_1,y_1).$Please help me.Thanks. 


Now $PA=\sqrt{(x-2)^2+(y+4)^2}$ and $PB=\sqrt{(x-4)^2+(y+2)^2}$  Now using Cosine formula
We get $$\displaystyle \cos \theta = \frac{(PA)^2+(PB)^2-(AB)^2}{2\cdot (PA)\cdot (PB)}$$
Now we know that $$\displaystyle |\cos \theta | \leq 1\Rightarrow \left|\frac{(PA)^2+(PB)^2-(AB)^2}{2\cdot (PA)\cdot (PB)}\right|\leq 1$$
So we get $$|(PA)^2+(PB)^2-(AB)^2|=2|PA||PB|\Rightarrow \left |PA-PB\right|^2\leq |AB|^2$$
So we get $|PA-PB|\leq |AB|$ and equality hold when $\theta = \pi$
Means $A,P,B$ are Collinear. 
So  equation of line $PAB$ is $$\displaystyle y+4 = \frac{-2+4}{4-2}(x-2)\Rightarrow x-y=6$$
Now solving $2x-y=-5$ and $x-y=6\;,$ we get $(x,y) = (-11,-17)$ 
