Maths-
General
Easy
Question
x2 + 20x + y2 + 16y = -20
The equation above defines a circle in the xy-plane. What are the coordinates of the center of the circle?
- (−20, −16 )
- (−10, −8)
- (10, 8)
- (20, 16)
The correct answer is: (−10, −8)
The standard equation of a circle is given by:
(x-h)2 + (y-k)2 = r2
Where (h,k) is the coordinates of center of the circle and r is the radius.
- We know that,
The standard equation of a circle in the xy-plane is of the form
(x − h)2 + (y − k)2 = r2 , where (h, k) are the coordinates of the center of the circle and r is the radius.
- We are given a equation of circle in xy plane ,
x2 + 20x + y2 + 16y = -20
- To convert the given equation to the standard form, first complete the squares. To do this we have to find the values which will be added to then complete the square .
- The first two terms need a 100 to complete the square, and the second two terms need a 64.
- Adding 100 and 64 to both sides of the given equation
(x2 + 20x +100) +(y2 + 16y+64) = -20+100+64
We can see first bracket is equivalent to (x+10)2 and second bracket is equivalent to (y+8)2
So, (x+10)2 + (y+8)2= 144
- Therefore , by comparing with the standard equation the coordinates of the center of the circle are
(h, k) = (−10, −8)
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