Find equation of tangent to the circle Coming back to this after about 6 months I now know how to solve it.
First I found the gradient of the radius $\frac{changeiny}{changeinx}$ >> $\frac{-8}{6}$ >> $1.33333333$
Then I found the negative reciprocal of the radius gradient to get the gradient of the tangent because it is at $90^\circ$ to the radius which becomes $\frac{1}{1.33333333}$ >> $0.75$
I then substituted the $x$ and $y$ values where the tangent touches the circle into the $y=mx+c$ equation >> $-8=0.75*6+c$ >> $-8=4.5+c$ >> $c=-12.5$
Which means that the equation of the tangent turned out to be .....  $y=0.75x-12.5$
ORIGINAL QUESTION:
The circle has the equation $x^2+y^2=100$
Find the equation of the tangent to the circle at the point $A(6,-8)$.
Any clues on how to solve this?


Alternatively if you haven't learnt calculus you can say there a line in the form $y=mx+c$  passing through the point $(6,-8)$ so $-8= 6m+c \implies c=-8-6m \implies y=mx+(-8-6m) $
Then substitute this into the circle equation:
$$ x^2+y^2=100 \implies x^2+(mx+(8-6m))^2=100 $$
$$ \Leftrightarrow x^2+m^2x^2+2mx(8-6m)+(8-6m)^2=100 \implies m^2 x^2 - 12 m^2 x + 36 m^2 + 16 m x - 96 m + x^2 + 64=100$$
$$ \Leftrightarrow x^2(m^2+1) +x(-12m^2+16m)+(36m^2-96m-36)=0$$
For tangency the discriminant of this equation must equal zero so 
$$ (-12m^2+16m)^2-4(m^2+1)(36m^2 - 96m-36) =0$$
$$ \Leftrightarrow 16(4m+3)^2=0 \implies m=\frac{-3}{4}$$
Hence the equation of the tangent line is 
$$ y=\frac{-3}{4}x - \frac{7}{2}$$
