A circle in the standard (x,y) coordinate plane has its center at (-4,3), and an area of 9π square coordinate units. Which of the following is an equation for the circle?

A. (x+4)^2-(y-3)^2 =81

B. (x-3)^2+(y+4)^2 =9

C. (x+4)^2+(y-3)^2 =9

D. (x-4)^2+(y-3)^2 =81

E. (x+4)^2+(y-3)^2 =81

Question

A circle in the standard (x,y) coordinate plane has its center at (-4,3), and an area of 9π square coordinate units. Which of the following is an equation for the circle?

A. (x+4)^2-(y-3)^2 =81

B. (x-3)^2+(y+4)^2 =9

C. (x+4)^2+(y-3)^2 =9

D. (x-4)^2+(y-3)^2 =81

E. (x+4)^2+(y-3)^2 =81

anonymous · Answer

B

cwrw238 · Answer

general form of circle equation is
(x -a)^2 + (x-b)^2 = r^2
where (a,b) is the center and r = radius

anonymous · Answer

Since the equation of a circle is (x-a)^2 + (x-b)^2 = r^2, where (a, b) is the centre, and area = pi r^2, we know that r is 3. So all we have to do is see which of B or C fits with the equation of a circle and we see that it is B.

cwrw238 · Answer

thats better traxter - an explanation of how you got there

jim_thompson5910 · Answer

it's not B

jim_thompson5910 · Answer

the center is (-4,3), so h = -4 and k = 3

anonymous · Answer

It is C

cwrw238 · Answer

yes

jim_thompson5910 · Answer

The general form is (x-h)^2+(y-k)^2=r^2

anonymous · Answer

What a shameful mistake

jim_thompson5910 · Answer

its ok

cwrw238 · Answer

thats not shameful!
- just human

anonymous · Answer

I mean....i was doing it to test the person asking the question of course!
But seriously, this highlights a mistake I see very often in my students. Be very careful not to mix up the x and y co-ordinates.
Also realise when you've been online too long and get to bed :P