Just one sec...
Hey! Sign in to get help with your study questionsNew here? Join for free to post

Circle Tangent questions.

Announcements Posted on
Results day: make sure you know what to expect before you get your grades. Here's how to be ready. 22-07-2016
    • Thread Starter

    I am having a fair bit of trouble getting the correct answers for a few questions in my math book

    Find the equations of the tangents to the circle x^2+y^2+6x-2y-15=0 that are perpendicular to the line 4x+3y+5=0

    Find the equations of the tangent fromt he given point to the given circle.

    Point (0,0) Circle x^2-y^2+4x+2y+4=0

    Tried to get the answer for these for the last half an hour but failed to do so.

    For the first one you are looking for a line

    You know the gradient of the line but not the intercept so you will still have +c in the equation

    Then you substitute that line into the circle equation ... then you will have a quadratic in x

    You are looking for a single solution so the discriminant = 0

    For the other one the meted is basically the same but in your equation of a line you know c but not m

    first find the general form of the perpandicular to the circle so you have ax+by=c.
    rearrange for y= whatever
    substitute y into your circle and you want the discriminant to be equal to 0, so that for the x there in only once y possible (the discriminant should be a quadractic in c so solve this equation)

    for this second one find the formula of the line first and you should get y=kx.
    substitute this in to find the two possible values of k using a similar method
    • Thread Starter

    Got the second question still having trouble on the 1st.

    (Original post by Sgany)
    Tried those methods and got no results so far.

    me neither

    is the question correct
    • Thread Starter

    (Original post by TenOfThem)

    me neither

    is the question correct
    Probably not the maths book is pretty crap and full of mistakes I will talk to my teacher tomorrow about it.

    The point (0,0) doesn't lie on the circle.

    EDIT: Whoops, wrong circle.
    EDIT2: Well the second one isn't even a circle, it's a hyperbola?
    EDIT3: Still doesn't lie on it even if it was a circle...

    (Original post by Sgany)
    I am having a fair bit of trouble getting the correct answers for a few questions in my math book

    Find the equations of the tangents to the circle x^2+y^2+6x-2y-15=0 that are perpendicular to the line 4x+3y+5=0
    Is line 4x+3y+5=0 a tangent to the circle? If so then a line perpendicular to it would go through the centre of the circle. You can use the equation of the circle to find that using completing the square. Then you just need to work out the gradient of the tangent to the circle and insert it all into y-y1=m(x-x1).
    Is this what you're looking for?


Submit reply


Thanks for posting! You just need to create an account in order to submit the post
  1. this can't be left blank
    that username has been taken, please choose another Forgotten your password?
  2. this can't be left blank
    this email is already registered. Forgotten your password?
  3. this can't be left blank

    6 characters or longer with both numbers and letters is safer

  4. this can't be left empty
    your full birthday is required
  1. Oops, you need to agree to our Ts&Cs to register
  2. Slide to join now Processing…

Updated: April 17, 2012
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

Are you currently in a relationship?
Useful resources

Make your revision easier


Maths Forum posting guidelines

Not sure where to post? Read here first


How to use LaTex

Writing equations the easy way

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22

Registered Office: International House, Queens Road, Brighton, BN1 3XE

Quick reply
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.