{"id":10162,"date":"2017-06-06t11:54:19","date_gmt":"2017-06-06t18:54:19","guid":{"rendered":"\/\/www.catharsisit.com\/hs\/?p=10162"},"modified":"2018-10-24t03:53:35","modified_gmt":"2018-10-24t10:53:35","slug":"ap-calculus-bc-review-parametric-functions","status":"publish","type":"post","link":"\/\/www.catharsisit.com\/hs\/ap\/ap-calculus-bc-review-parametric-functions\/","title":{"rendered":"ap calculus bc review: parametric functions"},"content":{"rendered":"
parametric functions only show up on the ap calculus bc exam. in this article we’ll take a close look at these kinds of functions which turn out to be extremely useful in the sciences.<\/p>\n
in fact, this is one case in which the phrase “it’s not rocket science!”<\/em> isn’t really appropriate. parametric equations play a huge role in rocket guidance systems!<\/p>\n while they may at first seem foreign and confusing, parametric functions are just a more flexible way to track motion in the plane.<\/p>\n usually you might think of a function as y<\/em> = f<\/em>(x<\/em>). the graph of a function must pass the vertical line test (vlt)<\/em>, and as a result, your options are limited. <\/p>\n sure you can use typical functions to define parabolas, catenaries and even sinusoidal waves, but what about a the orbit of a planet?<\/p>\n certainly any circular or elliptical graph fails the vlt. so you can’t write a function of the form y<\/em> = f<\/em>(x<\/em>) for this situation.<\/p>\n instead, you need to be able to specify both x<\/em> and y<\/em> in terms of some independent parameter<\/strong>, t<\/em>. that’s where the term parametric<\/strong> comes from.<\/p>\n a parametric function<\/strong> (or a set of parametric equations<\/strong>) is a pair of two functions specifying the x<\/em>– and y<\/em>-coordinates of a point moving through the plane.<\/p>\n think of each function as a separate control, one for x<\/em> and one for y<\/em>. perhaps the best physical example of parametric equations is the etch-a-sketch.<\/p>\n the etch-a-sketch has two knobs, one controlling the vertical, and one controlling the horizontal position of a stylus. when the two knobs work together, almost anything can be drawn!<\/p>\n there is a straightforward way to graph any parametric function. <\/p>\n graph the parametric function defined by x<\/em> = t<\/em>2<\/sup> – 2t<\/em> + 1 and y<\/em> = –t<\/em>2<\/sup> + 2.<\/p>\n because there was no range specified for t<\/em>, let’s just pick a few easy numbers to work with. remember, use both positive and negative values to get a good sense for how the function behaves. <\/p>\n i like to organize my work in a table.<\/p>\n <\/p>\n next, plot these points on a coordinate plane.<\/p>\n finally, connect the dots in order of increasing t<\/em>.<\/p>\n now that you have seen some graphing, let’s talk about slope. as you know, the derivative measures slope<\/strong>. but how do you find the derivative of a set parametric equations? <\/p>\n we must be careful, because there are two equations to deal with. should you take the derivative of f<\/em>(t<\/em>) or g<\/em>(t<\/em>)?<\/p>\n in fact, you’ll have to take the derivative of both<\/em>. here is the formula for dy<\/em>\/dx<\/em>.<\/p>\n what is the equation of the tangent line at t<\/em> = π\/6 for the parametric function x<\/em> = 3 cos t<\/em>, y<\/em> = 3 sin t<\/em>?<\/p>\n using the derivative formula, we get:<\/p>\n dy<\/em>\/dx<\/em> = (3 cos t<\/em>)\/(-3 sin t<\/em>) = -cos t<\/em> \/ sin t<\/em>.<\/p>\n plugging in t<\/em> = π\/6, the slope is -cos(π\/6)\/sin(π\/6) = -1.732.<\/p>\n now we also need to know what the x<\/em>– and y<\/em>-coordinates are for the point in question.<\/p>\n x<\/em> = 3 cos(π\/6) = 2.598, and y<\/em> = 3 sin(π\/6) = 1.5.<\/p>\n therefore, using the point-slope form, the equation of the tangent line is:<\/p>\n y<\/em> = -1.732(x<\/em> – 2.598) + 1.5<\/p>\n there is another interpretation of the derivative that allows you to compute the velocity<\/strong> of an object traveling along a parametric curve.<\/p>\n the idea is to think of a parametric function as a vector<\/em>. <\/p>\nwhat are parametric functions?<\/h2>\n
definition<\/h3>\n
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graphing<\/h3>\n
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example graph<\/h3>\n
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\n t<\/em><\/th>\n x<\/em> = t<\/em>2<\/sup> – 2t<\/em> + 1<\/th>\n y<\/em> = –t<\/em>2<\/sup> + 2<\/th>\n (x<\/em>, y<\/em>)<\/th>\n<\/tr>\n<\/thead>\n\n \n -3<\/td>\n 16<\/td>\n -7<\/td>\n (16, -7)<\/td>\n<\/tr>\n \n -2<\/td>\n 9<\/td>\n -2<\/td>\n (9, -2)<\/td>\n<\/tr>\n \n -1<\/td>\n 4<\/td>\n 1<\/td>\n (4, 1)<\/td>\n<\/tr>\n \n 0<\/td>\n 1<\/td>\n 2<\/td>\n (1, 2)<\/td>\n<\/tr>\n \n 1<\/td>\n 0<\/td>\n 1<\/td>\n (0, 1)<\/td>\n<\/tr>\n \n 2<\/td>\n 1<\/td>\n -2<\/td>\n (1, -2)<\/td>\n<\/tr>\n \n 3<\/td>\n 4<\/td>\n -7<\/td>\n (4, -7)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\n
<\/p>\n
derivatives of parametric functions<\/h2>\n
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example — tangent line<\/h3>\n
derivatives and velocity<\/h3>\n