(adsbygoogle = window.adsbygoogle || []).push({}); [SOLVED] Checking: Circle Equations Questions

Last time I got people to check my homework here, I got things marked wrong when they where said to be right. Lets see if it happens again...

1. The problem statement, all variables and given/known data

(1) The circle C has centre (3, 4) and passes through the point (8, 8).

Find an equation for C.

(2) Find the centre and radius of the circle x² + y² + 4x + 6y – 17 = 0

(3) A(4, 0) and B(3, 5) are the end points of a diameter of the circle C.

Find (a) the exact length of AB,

(b) the coordinates of the midpoint P of AB,

(c) an equation for the circle C.

2. Relevant equations

None

3. The attempt at a solution

Right here is my working for (1)

Since we know the circle centre and a point the circle passes through, I can use pythag to work on the radius right? It seems a little too simple to work, but I can't see anything wrong with it. After drawing a clear diagram;

(4+8)² + 5² = r²

r² = 144 + 25

r² = 169

r = 13

Therefore the equation must be (x-3)² + (y-4)² = 13

Correct?

Now, my working for (2);

The equation needs to be factorised, so I have to "complete the square" to get the stuff inside the brackets. This is usually where I go wrong, so check this bit carefully please!

(x+2)² - 4 + (y+3)² - 9 - 17 = 0

Rearraged:

(x+2)² + (y+3)² = 17

Ah, I have to go now, but I'll post the answers to 3 quickly:

a) I got 5.09

b) x = 3.5

y = 2.2

and

c) (x+3.5)² + (y+2.2)² = 5.09

Sorry about that lack of working out. Thank you for your time if you've read this far!

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Checking: Circle Equations Questions

**Physics Forums | Science Articles, Homework Help, Discussion**