Jim Lambers MAT 169 Fall Semester 2009-10 Lecture 32 Notes These notes correspond to Section 92 in the text Tangents of Parametric Curves When a curve is described by an equation of the form y = f(x), we know that the slope of the tangent line of the curve at the point (x 0, y 0 ) = (x 0, f(x 0 )) is given by = f (x) However, if the curve is defined by parametric equations x = f(t), y = g(t), then we may not have a description of the curve as a function of x in order to compute the slope of the tangent line in this way Instead, we apply the Chain Rule to obtain Solving for / yields = = This allows us to express / as a function of the parameter t Example The slope of the tangent to the spiraling curve defined by x = t sin t, y = t cos t, which is shown in the Lecture 31 notes, is given by = = cos t t sin t sin t + t cos t At the point (/2, 0), which corresponds to t = /2, the slope of the tangent is m = cos 2 2 sin 2 sin 2 + 2 cos 2 1 = 0 2 1 1 + 2 0 = 2
From the point-slope form of the equation of a line, we see the equation of the tangent line of the curve at this point is given by y 0 = ( x ) 2 2 We know that a curve defined by the equation y = f(x) has a horizontal tangent if / = 0, and a vertical tangent if f (x) has a vertical asymptote For parametric curves, we also can identify a horizontal tangent by determining where / = 0 This is the case whenever / = 0, provided that / = 0, thus excluding the case where / is the indeterminate form 0/0 Similarly, the tangent line is vertical whenever / = 0, but / = 0 Example Consider the unit circle, which can be parametrized by the equations x = cos t, y = sin t, 0 t < 2 The slope of the tangent at any point on the circle is given by = = cos t = cot t sin t A horizontal tangent occurs whenever cos t = 0, and sin t = 0 This is the case whenever t = /2 or t = 3/2 Substituting these parameter values into the parametric equations, we see that the circle has two horizontal tangents, at the points (0, 1) and (0, 1) A vertical tangent occurs whenever sin t = 0, and cos t = 0 This is the case whenever t = 0 or t = Substituting these parameter values into the parametric equations, we see that the circle has two vertical tangents, at the points (1, 0) and ( 1, 0) It is important to note that unlike a curve defined by y = f(x), a point on the curve may have more than one tangent line, because a parametric curve is allowed to intersect itself Example Consider the curve defined by the parametric equations x = t 2, y = (t 2 4) sin t This curve has two tangents at the point ( 2, 0) To see this, we first note that x = t 2 = when t = ± Substituting these values into the equation for y, we obtain y = 0, since sin t = 0 when t = ± Therefore, there are two distinct parameter values corresponding to this point on the curve Next, we must compute / for both values of t We have = = (t2 4) cos t + 2t sin t 2t 2 = sin t + t2 4 t cos t
Substituting t = yields = sin( ) + ( )2 4 cos( ) = 2 4 18684 On the other hand, substituting t = yields The curve is illustrated in Figure 1 = sin + 2 4 cos = 2 4 18684 Figure 1: Graph of the parametric curve x = t 2, y = (t 2 4) sin t In order to graph curves, it is helpful to know where the curve is concave up or concave down For a curve defined by y = f(x), this is determined by computing its second derivative d 2 y/ 2 = f (x) and checking its sign For a parametric curve, we can compute d 2 y/ 2 in the same way as /, by using the Chain Rule First, we note that d 2 y 2 = d 3 ( )
Then, from the Chain Rule, d ( ) = d ( ) = d2 y 2 Solving for d 2 y/ 2 yields d 2 y 2 = d ( ) To use this formula, one first computes / in terms of / and /, as described above Then, / is a function of t, which can be differentiated with respect to t in the usual way, before being divided by / to obtain d 2 y/ 2 It is possible to obtain a formula for d 2 y/ 2 that uses only derivatives of x and y with respect to t By applying the Quotient Rule to differentiate / with respect to t, we obtain d 2 d y 2 y 2 = 2 d 2 x 2 ( although the first formula may be easier to remember ) 3, Example Consider the astroid, illustrated in the Lecture defined by the parametric equations x = cos 3 t, y = sin 3 t, 0 t < 2 This curve is illustrated in Figure 2 To determine where the curve is concave up or concave down, we first compute / as a function of t: = = 3 sin2 t cos t 3 cos 2 = tan t t sin t Next, we use this to compute d 2 y/ 2 : ) d 2 y 2 = d ( = sec 2 t 3 cos 2 t sin t = 1 3 cos 4 t sin t We conclude that the astroid is concave up whenever sin t > 0, which is the case when y > 0 It is concave down whenever sin t < 0, which is the case whenever y < 0 4
Figure 2: Graph of the astroid x = cos 3 t, y = sin 3 t, for 0 t < 2 Areas Under Parametric Curves Recall that the area A of the region bounded by the curve y = F (x), the vertical lines x = a and x = b, and the x-axis is given by the integral b a F (x) Now, suppose that the curve y = F (x) is also defined by the parametric equations x = f(t), y = g(t), for α t β Furthermore, suppose that f(α) = a and f(α) = b If the curve is traversed only once as t increases from α to β, then the area can also be computed by integrating with respect to t as follows: b a F (x) = β α g(t)f (t) 5
On the other hand, if t = α corresponds to the right endpoint of the curve, and t = β corresponds to the left endpoint, then limits of integration must be reversed: α β β g(t)f (t) = g(t)f (t) α Example The upper half-circle with center (0, 0) and radius 1 can be defined by the parametric equations x = cos t, y = sin t, for 0 t Because t = 0 corresponds to the right endpoint of this curve, and t = corresponds to the left endpoint, the area bounded by the upper half-circle and the x-axis is given by 0 sin t( sin t) = 0 sin 2 t = 0 sin 2 t = which, as expected, is half of the area of the circle 0 1 cos 2t 2 = t 2 sin 2t 4 0 = 2, 6
Summary The slope of the tangent line of a parametric curve defined by parametric equations x = f(t), y = g(t) is given by / = (/)/(/) A parametric curve has a horizontal tangent wherever / = 0 and / = 0 It has a vertical tangent wherever / = 0 and / = 0 The concavity of a parametric curve at a point can be determined by computing d 2 y/ 2 = d(/)//(/), where / is best represented as a function of t, not x The curve is concave up when d 2 y/ 2 is positive, and concave down if it is negative A parametric curve x = f(t), y = g(t) can have two tangents at a point (x 0, y 0 ) on its graph, if there are two distinct values of the parameter t, t 1 and t 2, such that f(t 1 ) = f(t 2 ) = x 0 and g(t 1 ) = g(t 2 ) = y 0 The area of the region bounded by the parametric curve x = f(t), y = g(t), the x-axis, the line x = a, and the line x = b, where f(α) = a and g(β) = b, is the integral from α to β of g(t)f (t), provided that the curve is only traversed once as t increases from α to β 7