oh yeah and k would be the gradient
Hope im in the right forum post
Basically i have to prove algebraically- the values of k in y=kx+7 that do no intersect the circle x2 +y2=25
What i already know -In my text book i learnt that the radius of the circle is sqrt 25=5
so radius all the way around the circle is 5.
The line y=kx+7 has a y intercept of 7, where it cuts the y axis.
Im not sure about how to go on proving this algebraically, i read an article that you have to sustitute the line equation into the circle one on this site :BBC - Higher Bitesize Maths - The circle : Page 5
Yes, to investigate intersections sub y=kx+7 into x^2+y^2=25
So x^2+(kx+7)^2=25 x^2+k^2x^2+14kx+49-25=0
Arrange as a quadratic in x Get (1+k^2)x^2+14kx+24=0
We want this to have no real roots. (The condition for ax^2+bx+c=0 to have no real roots is b^2-4ac<0). Try and apply this here.
is this applying it ?
a b c
b^2-4ac<0 then sub into it (1+k^2)x^2+14kx+24=0
making it :
196- 96 +96k^2<0
Am i making progress? or have i done it completely wrong?
also why do we want it to have no real roots ? what do real roots do?
but then what are all the values that miss the circle - from the closest possible to the circle , then outward for 'k' going back to my main question. K is the gradient that controls the slope of the line - so how do i find all the values of k that do not intersect the circle given the equation of the line is y=kx+7 and the circle is x^2 + y^2=25.
You are using the correct method but your 3rd line should have been (14k)^2-4(1+k)24<0 Yo left out the 1st k.
If there were some roots these would be the x coordinates of the points of intersection. So no roots means no intersections.
Thank biffboy for the reply - much appreciated
i probably worded a bit wrong at the start but i wanted to find out all the values of k that do not intersect the circle, given that k is the gradient- changing the slope of the line . Im sure its gonna be an answer like all values of must k < ? to not intersect the circle. But i need to prove it algebraically. would the above answer be on the right track to solving the problem? If not could you show me how or hint me towards what i need to do to get the answer thanks so much
made the change
196k- 96 +96k^2<0
(k )(k )
or if cant find 2 factors use quadratic formula
ill post to you tommorrow coz i need to get some rest- thanks for the help so far
i think because "k" is any value that does not make the line intersect/ cross with the circle- thats probably why theres and error - still need to find the range of values that for k that does not cross the circle
then factorise or use quadratic formula?
In your 2nd line that 196k should be 196k^2
So 196k^2-96-96k^2<0 So 100k^2<96 So k^2<96/100 So K^2<0.96 So -root 0.96<x<+root0.96
Note for root(96/100) the book has probably done these steps. root(96/100)=root96/root100=root(16x6)/10=(4root6)/10 =(2root6)/5
Thanks again biffboy! im grateful for youre help
im making simple mistakes - but thanks for correcting me.
so once i get the answer would all the range of values for k be -root 0.96<x<+root0.96 meaning between -root0.96 and +root0.96 would be all the values for k- the gradient?
Remembering the question was "Find the range of values for "k" , for which the circle x^2 + y^2=25 and the line y=kx+7 do not intersect"