Doing a core 2 maths question, realised i cant remember how to find where a tangient and circle meet.(adsbygoogle = window.adsbygoogle || []).push({});

The circle equation provided in question was x^2 + y^2 -10x + 9 = 0

same as (x - 5)^2 + y^2 = 4^2

Question was:

"Given that line l with gradient 7/2 is a tangient to the circle, and that l touches circle at point T

find an equation that passes through the centre of the circle and T"

i tried to find out where line and circle met but wasnt able too. In mark scheme they had a very easy way to do it (m1m2=-1 so gradient is -2/7 and you know the co-ordinates of the centre of the circle, so you use y-y1 = m(x-x1) )

So, anyway i tried to put line and circle together and realised i couldnt... this is what i did how do you do it?

What i first was say that the forumula of the straight line is 7/2x + c = y where c is a constant

i then substituted that in the circle forumula to get (x-5)^2 + (7/2x + c)^2 = 4^2

multiplied out to get (53/4)x^2 + (7c - 10)x + 9 + c^2 = 0

since there can only be one result, b^2 - 4ac = 0 so

(7c -10)^2 - 4 * (53/4) * (9 + c^2) = 0

which ends up with

102c^2 -140c + 577 = 0

which does not have a result...

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# Homework Help: Circle and tangient question, AS core 2

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