// Numbas version: exam_results_page_options {"name": "Parametric representations of a curve - find tangent and length", "extensions": [], "custom_part_types": [], "resources": [], "navigation": {"allowregen": true, "showfrontpage": false, "preventleave": false, "typeendtoleave": false}, "question_groups": [{"pickingStrategy": "all-ordered", "questions": [{"variable_groups": [], "variables": {"t2": {"templateType": "anything", "group": "Ungrouped variables", "definition": "random(1,2,4)", "description": "", "name": "t2"}, "b": {"templateType": "anything", "group": "Ungrouped variables", "definition": "random(1..9)", "description": "", "name": "b"}, "a": {"templateType": "anything", "group": "Ungrouped variables", "definition": "random(2..9)", "description": "", "name": "a"}}, "ungrouped_variables": ["a", "b", "t2"], "question_groups": [{"pickingStrategy": "all-ordered", "questions": [], "name": "", "pickQuestions": 0}], "name": "Parametric representations of a curve - find tangent and length", "functions": {}, "showQuestionGroupNames": false, "parts": [{"scripts": {}, "gaps": [{"answer": "{-a*b}*sin({b}*t)", "showCorrectAnswer": true, "vsetrange": [0, 1], "checkingaccuracy": 0.001, "checkvariablenames": false, "expectedvariablenames": [], "showpreview": true, "checkingtype": "absdiff", "scripts": {}, "type": "jme", "answersimplification": "all", "marks": 1, "vsetrangepoints": 5}, {"answer": "{-a*b}*cos({b}*t)", "showCorrectAnswer": true, "vsetrange": [0, 1], "checkingaccuracy": 0.001, "checkvariablenames": false, "expectedvariablenames": [], "showpreview": true, "checkingtype": "absdiff", "scripts": {}, "type": "jme", "answersimplification": "all", "marks": 1, "vsetrangepoints": 5}], "type": "gapfill", "prompt": "

Find the tangent vector $\\boldsymbol{u}$ to the curve.

$\\boldsymbol{u}=($[[0]]$,$[[1]]$)$. (Do not enter decimals in your answers.)

", "showCorrectAnswer": true, "marks": 0}, {"scripts": {}, "gaps": [{"answer": "({2*a*b}/{t2})*pi", "vsetrange": [0, 1], "checkingaccuracy": 0.001, "showCorrectAnswer": true, "expectedvariablenames": [], "notallowed": {"message": "

Do not enter decimals in your answer.

Find the length of the curve $s$, given $t_1=-\\simplify{1/{t2}}$ and $t_2=\\simplify{1/{t2}}$.

$s=$ [[0]]. (Enter your answer as a fractional multiple of $\\pi$. Do not enter decimals.)

", "showCorrectAnswer": true, "marks": 0}, {"scripts": {}, "gaps": [{"answer": "{a}*cos(s/{a}-{b}*pi/{t2})", "vsetrange": [0, 1], "checkingaccuracy": 0.001, "showCorrectAnswer": true, "expectedvariablenames": [], "notallowed": {"message": "

Enter your answer as a fractional multiple of $\\pi$. Do not enter decimals.

", "showStrings": false, "partialCredit": 0, "strings": ["."]}, "showpreview": true, "checkingtype": "absdiff", "scripts": {}, "checkvariablenames": false, "type": "jme", "answersimplification": "all", "marks": 1, "vsetrangepoints": 5}, {"answer": "{-a}*sin(s/{a}-{b}*pi/{t2})", "vsetrange": [0, 1], "checkingaccuracy": 0.001, "showCorrectAnswer": true, "expectedvariablenames": [], "notallowed": {"message": "

Enter your answer as a fractional multiple of $\\pi$. Do not enter decimals.

Find another parametric representation of the curve, again with $t_1=-\\simplify{1/{t2}}$ and $t_2=\\simplify{1/{t2}}$, using $s$ as the curve parameter, such that $0\\leqslant s\\leqslant \\simplify{{2*a*b}/{t2}}\\pi$.

$s\\rightarrow($[[0]]$,$[[1]]$)$. (Enter your answers as fractional multiples of $\\pi$. Do not enter decimals.)

", "showCorrectAnswer": true, "marks": 0}], "statement": "

You are given the curve $t\\rightarrow\\pmatrix{\\var{a}\\cos(\\simplify{{b}t}),\\var{-a}\\sin(\\simplify{{b}t})}$, where $t_1\\pi\\leqslant t\\leqslant t_2\\pi$.

", "tags": ["checked2015", "MAS2104"], "rulesets": {}, "preamble": {"css": "", "js": ""}, "type": "question", "metadata": {"notes": "", "licence": "Creative Commons Attribution 4.0 International", "description": "

Calculation of the length and alternative form of the parameteric representation of a curve, involving trigonometric functions.

"}, "variablesTest": {"condition": "", "maxRuns": 100}, "advice": "

The tangent vector to the curve $t\\rightarrow\\pmatrix{x,y}$ is given by $\\boldsymbol{u}\\equiv\\pmatrix{\\frac{\\mathrm{d}x}{\\mathrm{d}t},\\frac{\\mathrm{d}y}{\\mathrm{d}t}}$.

The length $s$ of the curve in the range $t_1\\pi\\leqslant t\\leqslant t_2\\pi$ is given by

\\[s=\\int_{t_1\\pi}^{t_2\\pi}{u\\mathrm{d}t},\\]

where $u^2=\\lvert\\boldsymbol{u}\\rvert^2=\\boldsymbol{u\\cdot u}$.

In this question, therefore, $\\boldsymbol{u}=\\pmatrix{\\frac{\\mathrm{d}}{\\mathrm{d}t}\\left\\{\\var{a}\\cos(\\simplify{{b}*t})\\right\\},\\frac{\\mathrm{d}}{\\mathrm{d}t}\\left\\{\\var{-a}\\sin(\\simplify{{b}*t})\\right\\}}=\\pmatrix{\\var{-a*b}\\sin(\\simplify{{b}*t}),\\var{-a*b}\\cos(\\simplify{{b}*t})}$, and so $u^2=\\var{(a*b)^2}$.

Then

\\[s=\\int_{t_1\\pi}^{t_2\\pi}{u\\mathrm{d}t}=\\var{a*b}\\int_{t_1\\pi}^{t_2\\pi}{\\mathrm{d}t}=\\var{a*b}(t_2-t_1)\\pi.\\]

Finally, substitute $t_1=-\\simplify{1/{t2}}$ and $t_2=\\simplify{1/{t2}}$ into this expression for $s$, to find the length of the curve over the given range of $t$.

Hence $s=\\simplify{{2*a*b}/{t2}}\\pi$.

An alternative parametric representation, using $s$ as the curve parameter is given by

\\[s=\\int_{t_1\\pi}^{t}{u\\mathrm{d}\\tau}=\\var{a*b}\\int_{t_1\\pi}^{t}{\\mathrm{d}\\tau}=\\var{a*b}(t-t_1)\\pi.\\]

Now rearrange this expression for $t(s)$, so

\\[t(s)=\\frac{s}{\\var{a*b}}+t_1\\pi,\\]

and substitute into the original representation of the curve $t\\rightarrow\\pmatrix{\\var{a}\\cos(\\simplify{{b}t}),\\var{-a}\\sin(\\simplify{{b}t})}$ with $t_1\\pi\\leqslant t\\leqslant t_2\\pi$. Hence

\\[s\\rightarrow\\pmatrix{\\var{a}\\cos\\left(\\simplify{s/{a}}+\\var{b}t_1\\pi\\right),\\var{-a}\\sin\\left(\\simplify{s/{a}}+\\var{b}t_1\\pi\\right)},\\]

with $0\\leqslant s\\leqslant\\var{a*b}(t_2-t_1)\\pi$.

Finally, substitute $t_1=-\\simplify{1/{t2}}$ and $t_2=\\simplify{1/{t2}}$ into the above expressions, to find the specific parametric representation corresponding to the given range of t:

\\[s\\rightarrow\\pmatrix{\\var{a}\\cos\\left(\\simplify{s/{a}}-\\simplify{{b*pi}/{t2}}\\right),\\var{-a}\\sin\\left(\\simplify{s/{a}}-\\simplify{{b*pi}/{t2}}\\right)},\\]

with $0\\leqslant s\\leqslant \\simplify{{2*a*b}/{t2}}\\pi$.

", "contributors": [{"name": "Newcastle University Mathematics and Statistics", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/697/"}]}]}], "contributors": [{"name": "Newcastle University Mathematics and Statistics", "profile_url": "https://numbas.mathcentre.ac.uk/accounts/profile/697/"}]}