// Numbas version: exam_results_page_options {"name": "Power series solution of second order ODE", "extensions": [], "custom_part_types": [], "resources": [], "navigation": {"allowregen": true, "showfrontpage": false, "preventleave": false, "typeendtoleave": false}, "question_groups": [{"pickingStrategy": "all-ordered", "questions": [{"variable_groups": [], "variables": {"b1": {"templateType": "anything", "group": "Ungrouped variables", "definition": "random(1..3)*sign(random(-1,1))", "name": "b1", "description": ""}, "a1": {"templateType": "anything", "group": "Ungrouped variables", "definition": "random(1..3)*sign(random(-1,1))", "name": "a1", "description": ""}, "amp2": {"templateType": "anything", "group": "Ungrouped variables", "definition": "'$a_{m+2}$'", "name": "amp2", "description": ""}}, "ungrouped_variables": ["a1", "amp2", "b1"], "rulesets": {}, "name": "Power series solution of second order ODE", "functions": {"addsumfunction": {"type": "string", "language": "javascript", "definition": "Numbas.jme.display.texOps.sum=function(thing,texArgs) {\n var lowerLimit = '';\n var upperLimit = '';\n if (texArgs[1]) {\n lowerLimit = texArgs[1];\n }\n if (texArgs[2]) {\n upperLimit = texArgs[2];\n }\n return ('\\\\sum_{'+lowerLimit+'}^{'+upperLimit+'}{'+texArgs[0]+'}');\n }\n return ('');", "parameters": []}}, "showQuestionGroupNames": false, "parts": [{"scripts": {}, "gaps": [{"answer": "{-b1}*a0/2", "showCorrectAnswer": true, "vsetrange": [0, 1], "scripts": {}, "checkvariablenames": false, "expectedvariablenames": [], "notallowed": {"message": "
Do not enter decimals in your answer.
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and a1
for $a_0$ and $a_1$ respectively. In addition, do not enter decimals.
$a_2=$ [[0]].
\n$a_3=$ [[1]].
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\nSeek a power series solution, about $x=0$, in the form
\n\\[y(x)=\\sum_{n=0}^{\\infty}{a_nx^n},\\]
\nof the differential equation
\n\\[\\simplify{y''+{a1}*x*y'+{b1}*y}=0.\\]
\nTake $a_0$ and $a_1$ to be arbitrary constants, and enter the coefficients $a_2$ and $a_3$ as functions of $a_0$ and $a_1$.
", "tags": ["checked2015", "MAS2105"], "question_groups": [{"pickingStrategy": "all-ordered", "questions": [], "name": "", "pickQuestions": 0}], "preamble": {"css": "", "js": ""}, "type": "question", "metadata": {"notes": "Uses a custom function to allow simplification of a LaTeX sum, in the same manner as e.g. int()
or defint()
.
Power series solution of $y''+axy'+by=0$ about $x=0$.
"}, "variablesTest": {"condition": "", "maxRuns": 100}, "advice": "We have
\n\\[y(x)=\\sum_{n=0}^{\\infty}{a_nx^n},\\]
\nso
\n\\[y'(x)=\\sum_{n=1}^{\\infty}{a_nnx^{n-1}},\\]
\nand
\n\\[y''(x)=\\sum_{n=2}^{\\infty}{a_nn(n-1)x^{n-2}}.\\]
\nSubstitute these expressions into the original differential equation to obtain
\n\\[\\simplify{sum(a_n*n*(n-1)x^(n-2),n=2,infty)+{a1}*sum(a_n*n*x^n,n=1,infty)+{b1}*sum(a_n*x^n,n=0,infty)}=0.\\]
\nNow reset the index $m=n-2$ in the first summation, and $m=n$ in the second and third summations to obtain
\n\\[\\simplify{sum({amp2}*(m+2)*(m+1)x^m,m=0,infty)+{a1}*sum(a_m*m*x^m,m=1,infty)+{b1}*sum(a_m*x^m,m=0,infty)}=0.\\]
\nThis equation must be valid for all values of $x$, so the coefficients of like powers of $x$ must vanish. Take $m=0$ to obtain the coefficients of $x^0$, then
\n\\[\\simplify{2*a2+{b1}*a0}=0,\\]
\nand so
\n\\[a_2=\\simplify{{-b1}*a0/2}.\\]
\nNow take $m=1$ to obtain the coefficients of $x^1$, so
\n\\[\\simplify{6*a3+{a1}*a1+{b1}*a1}=0,\\]
\nthen
\n\\[a_3=\\simplify{-{a1+b1}*a1/6}.\\]
", "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/"}]}