1. Circles HW #1

2. Complete the square and write as a squared binomial.
1. x2 + 6x + _____ = _____________
2. x2 – 10x + _____ = _____________
3. x2 + 24x + _____ = _____________
4. x2 + 2x + _____ = _____________
5. x2 – 5x + _____ = ______________ 6.
This will be a critical skill we will use later on with conic sections!

3. Solve by completing the square:
x2 + 6x – 16 = 0
x2 – 4x = 11

4. Write the equation in standard form for the given circle:

5. Lines Tangent to Circles
Section: 5.2
(Green Book)
Circles Quiz: Thursday Circles Test: Sept. 17

6. What we should know already:
Chord: A line segment whose endpoints are on the circle
Secant: A line that intersects the circle in two points
Radius: The distance from the center to a point on the circle

7. What we should know already:
Diameter: A chord that passes through the center of the circle
Tangent: A line in the plane of a circle that intersects the circle in EXACTLY one point

8. Helpful Theorems
If a line is tangent to a circle, then it is perpendicular to the radius drawn to the point of tangency

9. Helpful Theorems
In a plane, if a line is perpendicular to a radius of a circle at its endpoint on the circle, then the line is tangent to the circle.

10. If two lines are Perpendicular…
What do we know about their slopes?
Their slopes are opposite reciprocals to each other!!!!!!!!
Let’s use this to find some tangents….

11. Find an equation of the line tangent to the given circle at the given point
What’s the slope of the radius to point (-1, 3)?
So……..

12. Slope of Tangent
The slope of the tangent must be the opposite reciprocal of -3, which is

13. Writing the Equation Point Slope Form:
So to write the equation of our tangent line:

14. OR You can use Slope-Intercept form:
y = 1/3x + b  plug in the point,(-1, 3) to find b

15. Find an equation of the line tangent to the given circle at the given point
(x – 3)2 + (y + 2)2 = 130
at point (-4, 7)

16. Write a circular model Cell Phones
A cellular phone tower services a 10 mile radius. You get a flat tire 4 miles east and 9 miles north of the tower. Are you in the tower’s range?
SOLUTION
In the diagram above, the origin represents the tower and the positive y-axis represents north.
STEP 1
Write an inequality for the region covered by the tower. From the diagram, this region is all points that satisfy the following inequality:
x2 + y2 < 102

17. Write a circular model continued….
STEP 2
Substitute the coordinates (4, 9) into the inequality from Step 1.
x2 + y2 < 102
Inequality from Step 1
< 102 ?
Substitute for x and y.
97 < 100 
The inequality is true.
ANSWER
So, you are in the tower’s range.

18. HOMEWORK
Circle Homework #2 WS