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Polar Coordinates. We Live on a Sphere. Polar Coordinates. Up till now, we have graphed on the Cartesian plane using rectangular coordinates In the rectangular coordinate system a point is plotted as (x, y). Polar Coordinate.
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Polar Coordinates We Live on a Sphere
Polar Coordinates • Up till now, we have graphed on the Cartesian plane using rectangular coordinates • In the rectangular coordinate system a point is plotted as (x, y).
Polar Coordinate • In a polar coordinate system, we select a pint, called to pole, and then a ray with vertex at the pole, called the polar axis. • We still use an ordered pair to graph. • The new ordered pair is (r, θ). • If r > 0, then r is the distance of the point from the pole (like the origin)
Polar Coordinates • θ is the angle (in degrees or radians) formed by the polar axis and a ray from the pole. • We call the ordered pair (r, θ) the polar coordinates of the point.
Polar Coordinates • Since angles have several different ways to name them, there are an infinite number of polar coordinates for each point. (Unlike rectangular coordinates which have only one name for point on the Cartesian plane.)
Polar Coordinates • Find four names for the point • We are given a positive radius and a positive angle. We want to find a positive angle and a negative r, a pos r and neg angle, and a neg angle and neg r.
Steps for finding other polar coordinates 1. Subtract 360o (or 2p) to get a negative angle 2. Add 180o (or p) to change the r to negative (half-way around the circle to be on the other side of the polar graph) 3. Add or subtract 360o (or p) to find the other angle
Example • Graph • Examples
Conversion from Polar Coordinates to Rectangular Coordinates • If P is a point with polar coordinates (r, q), the rectangular coordinates • (x, y) of P are given by • x = r cos θ y = r sin θ • Remember
Examples • Find the rectangular coordinates of the points with the following polar coordinates:
Polar to Rectangular Coordinates • You can check your answers using your calculator. • First do 2nd APPS • Choose • Put in polar coordinates • Hit enter
Steps for Converting from Rectangular to Polar Coordinates • 1. Always plot the point (x, y) first • 2. To find r, r2 = x2 + y2 (Look familiar?) • 3. To find q, remember that we only know x and y. Therefore, the trig value that we can use involves only x and y – tangent.
Converting from Rectangular Coordinates to Polar Coordinates • Find polar coordinates of a point whose rectangular coordinates are • a. (0, 3) • b. (2, -2) • c. (-3, 3)
Transforming an Equation from Polar to Rectangular Form • Transform the equation r = cos q. • We do not have a formula for just • cos q, but we do have one for r cos q. • Multiply both sides by r to get r cos q. • That gives us r2 = r cos q
Transforming an Equation from Polar to Rectangular Form • r2 = x2 + y2 and r cos q = x • So, x2 + y2 = x • This is the equation of a circle • Find the answer by completing the square
Transforming an Equation from Polar to Rectangular Form • (x2 – x ) + y2 = 0 (complete the square) • (x2 – x + ¼) + y2 = ¼ • (x - ½)2 + y2 = ¼ • This is a circle whose center is (½, 0) and whose radius is ½
Transforming from Rectangular to Polar Form • y2 = 2x • (r sin q)2 = 2 r cos q • r2 sin2q = 2r cos q
Examples • More Examples • Tutorials
