Join TSR now and get all your revision questions answeredSign up now

C4 implicit Watch

    • Thread Starter
    Offline

    1
    ReputationRep:
    At what points are the tangents to the circle x^2+y^2-6y-8x=0 parallel to the y-axis?

    i differentiated the equation and got:

    2x + 2ydy/dx - 6dy/dx -8

    then my gradient equals this: 8-2x/2y-6

    now what do i do if i equal it to zero one of my co-ordinates my x coordinate will equal 4 and thats not in the answers.

    thanks in advance, kind regards
    Offline

    2
    ReputationRep:
    the first part is wrong it 2x+2ydy/dx not minus
    • Community Assistant
    Offline

    3
    ReputationRep:
    You have an incorrect sign in your differentiated line and forgot = 0

    You are not trying to find when dy/dx = 0

    You are trying to find when the tangent is parallel to the y axis not the x axis
    Offline

    0
    ReputationRep:
    You are trying to find when the line is parallel to the y axis so use dx/dy=0

    so (2y-6)/(2x-8)=0

    2y-6=0
    y=3

    When y=3, x^2-8x-9=0
    so (x-9)(x+1)=0

    voila (9,3) and (-1,3)

    Are these right?
    • Thread Starter
    Offline

    1
    ReputationRep:
    (Original post by BigBrain)
    You are trying to find when the line is parallel to the y axis so use dx/dy=0

    so (2y-6)/(2x-8)=0

    2y-6=0
    y=3

    When y=3, x^2-8x-9=0
    so (x-9)(x+1)=0

    voila (9,3) and (-1,3)

    Are these right?
    yessss you genuissssss
    Offline

    1
    ReputationRep:
    The gradient is undefined when denominator=0

    dy/dx= (2x-8)/(2y-6)=0

    Solve the denominator for 0.
 
 
 
Poll
Is GoT overrated?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

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

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.