// Numbas version: exam_results_page_options {"name": "Separable first order ODE with boundary condition", "extensions": [], "custom_part_types": [], "resources": [], "navigation": {"allowregen": true, "showfrontpage": false, "preventleave": false, "typeendtoleave": false}, "question_groups": [{"pickingStrategy": "all-ordered", "questions": [{"variable_groups": [], "variables": {"b": {"group": "Ungrouped variables", "templateType": "anything", "definition": "random(1..9)", "description": "", "name": "b"}, "n": {"group": "Ungrouped variables", "templateType": "anything", "definition": "random(1..4)", "description": "", "name": "n"}, "a": {"group": "Ungrouped variables", "templateType": "anything", "definition": "random(1..9)", "description": "", "name": "a"}}, "ungrouped_variables": ["a", "b", "n"], "question_groups": [{"pickingStrategy": "all-ordered", "questions": [], "name": "", "pickQuestions": 0}], "name": "Separable first order ODE with boundary condition", "functions": {}, "showQuestionGroupNames": false, "parts": [{"scripts": {}, "gaps": [{"answer": "({(b ^ (n + 1))} + ({(a * (n + 1))} * t)) ^ (1 / {(n + 1)}) - {b}", "vsetrange": [0, 1], "checkingaccuracy": 0.0001, "answersimplification": "std", "expectedvariablenames": [], "notallowed": {"message": "
Input all numbers as integers or fractions.
", "showStrings": false, "partialCredit": 0, "strings": ["."]}, "showpreview": true, "checkingtype": "absdiff", "scripts": {}, "checkvariablenames": false, "type": "jme", "showCorrectAnswer": true, "marks": 3, "vsetrangepoints": 5}], "type": "gapfill", "prompt": "The thickness at time $t$ is given by:
\n$x(t)=\\;\\;$[[0]]
\nInput all numbers as integers or fractions – not as decimals.
", "showCorrectAnswer": true, "marks": 0}], "variablesTest": {"condition": "", "maxRuns": 100}, "statement": "The thickness of ice on water, $x(t)$, grows according to the equation:
\\[\\frac{dx}{dt}=\\simplify[std]{{a}/(x+{b})^{n}}\\]
Given that $x(0)=0$ find $x(t)$.
29/06/2012:
\nAdded and edited tags.
\nChecked answer. Checking range OK as we are taking roots of positive numbers, given the choice of ranges for the variables.
\n18/07/2012:
\nAdded description.
\n23/07/2012:
\nAdded tags.
\nThe arbitrary constant A should be relabelled as A_1 in the Advice section part way though the solution.
\nQuestion appears to be working correctly.
\n", "licence": "Creative Commons Attribution 4.0 International", "description": "
Solve for $x(t)$, $\\displaystyle\\frac{dx}{dt}=\\frac{a}{(x+b)^n},\\;x(0)=0$
"}, "advice": "On rearranging the equation we get $\\displaystyle{\\simplify[std]{(x+{b})^{n}*(dx/dt) = {a}}}$ and on integrating we obtain:
$\\displaystyle{\\simplify[std]{(x+{b})^{n+1}/{n+1}={a}t +A} \\Rightarrow x+\\var{b}=(A+\\var{a*(n+1)}t)^{1/\\var{n+1}}}$
Using the condition $x(0)=0$ gives $\\displaystyle{A^{1/\\var{n+1}}=\\var{b} \\Rightarrow A=\\var{b^(n+1)}}$
\nHence the solution is:
\\[x(t) = \\simplify[std]{({(b ^ (n + 1))} + ({(a * (n + 1))} * t)) ^ (1 / {(n + 1)}) - {b}}\\]