# Homework Help: Intersection of tangents

1. Oct 1, 2012

### utkarshakash

1. The problem statement, all variables and given/known data
Two points represented by $z_{1}, z_{2}$ lie on circle |z|=1, the tangents to the circle at these points meet at the point represented by

2. Relevant equations

3. The attempt at a solution
Let the tangents meet at the point $z_{3}$. The centre of the given circle is (0,0).
∴ $z_{1}, z_{2}, z_{3} and 0$ are concyclic. Now what to do next?

2. Oct 1, 2012

### ehild

It is not true. z1 and z2 are on the circle which centre is (0,0) z3 can not lie on the circle.

Find the equations of the tangent line and the point where they cross each other.

ehild

3. Oct 2, 2012

### utkarshakash

I'm not saying that $z_{3}$ will lie on the circle with centre (0,0), Instead the 4 points which I mentioned will lie on some other circle and the centre of that circle will be the mid-point of line joining $z_{3}$ and 0.

4. Oct 2, 2012

### ehild

Sorry, I misunderstood you. You are right, z1,z2, (0,0)and z3 are on a circle. Find z3 in terms of z1 and z2. One possible way is to write up the equations of the tangent lines and find the intersection. Do you know the equation of a straight line in term of its normal?

ehild

Last edited: Oct 3, 2012
5. Oct 3, 2012

### utkarshakash

I know how to write equation of tangents in terms of x and y but not in the form of z. But I know the equation of a complex line.

6. Oct 4, 2012

### ehild

A complex number has x and y components like a vector in a plane. Write up the equations of the tangents in terms of x1,y1 and x2,y2. The coordinates of the point of intersection are x3,y3. You can convert it to the complex number z3=x3+iy3.

ehild

7. Oct 4, 2012

### utkarshakash

But I have to give the answer in terms of z1 and z2 and converting the answer to the required form will be calculative. Any other methods or should I go with this?

8. Oct 4, 2012

### ehild

Do it as you can and we will see.

ehild

9. Oct 7, 2012

### utkarshakash

After writing the equation in terms of x and y and then simultaneously solving the two lines I get

$\LARGE z_{3}= \frac{y_{2}-y_{1}}{x_{1}y_{2}-x_{2}y_{1}} + i \frac{x_{1}-x_{2}}{x_{1}y_{2}-x_{2}y_{1}}$

Now how do I represent this in terms of z1 and z2?

10. Oct 7, 2012

### ehild

z=x+iy. x=Re(z) or (z+z*)/2; y=Im(z) or y=(z-z*)/2i. (* means complex conjugate)

ehild

11. Oct 8, 2012

### utkarshakash

After simplification I get

$\LARGE z_{3}= \frac{2(z_{2}-z_{1}- \overline{z_{1}})}{\overline{z_{1}}z_{2}-z_{1} \overline{z_{2}}}$

But this is not the correct answer.

12. Oct 8, 2012