Question

Find the value of $x$, so that the points $(x,-1),(2,1)$ and $(4,5)$ are collinear.

Answer 1

Line from (2,1) & (4,5)

$(y-1)=\frac{5-1}{4-2}(x-2)\Rightarrow (y-1)2=4(x-2)$

$y-1=2x-4\Rightarrow y-2x+3=0$

$\therefore (x,-1)$ must satisfy $\Rightarrow -1-2x+3=0$

$\therefore 2x=2\Rightarrow x=1$