College Trigonometry, 2011a

942 Applications of Trigonometry
[ ]
0,
π
2 , we have that the curve begins to retrace itself at this point. Proceeding further, we find
that when 3π 2
≤ θ ≤ 2π, we retrace the portion of the curve in Quadrant IV that we first traced
out as π 2
≤ θ ≤ π. The reader is invited to verify that plotting any range of θ outside the interval
[0,π] results in retracting some portion of the curve. 4 We present the final graph below.
6
r
y
3
3
π
2
π
θ
3 6
x
−3
−3
−6
r =6cos(θ) intheθr-plane
r = 6 cos(θ) inthexy-plane
Example 11.5.2. Graph the following polar equations.
1. r =4− 2sin(θ) 2. r = 2 + 4 cos(θ) 3. r =5sin(2θ) 4. r 2 =16cos(2θ)
Solution.
1. We first plot the fundamental cycle of r =4− 2sin(θ) ontheθr-axes. To help us visualize
what is going on graphically, we divide up [0, 2π] into the usual four subintervals [ 0, π ] [
2 , π
[ ] [ 2 ,π] ,
π,
3π
2 and 3π
2 , 2π] , and proceed as we did above. As θ ranges from 0 to π 2
, r decreases from
4 to 2. This means that the curve in the xy-plane starts 4 units from the origin on the positive
x-axis and gradually pulls in towards the origin as it moves towards the positive y-axis.
r
y
6
θ runs from 0 to π 2
4
x
2
π
2
π
3π
2
2π
θ
4 The graph of r =6cos(θ) looks suspiciously like a circle, for good reason. See number 1a in Example 11.4.3.