1. Find the radius of curvature (ROC) at (a cos3 θ, a sin3
θ) on the curve x
2/3 +
y
2/3 = a
2/3
2. Find the radius of curvature (ROC) for the curve x = a(cos θ + θ sin θ), y =
a(sin θ − θ cos θ).
1.
The parametric equations are
"y=a\\sin^3 \\theta=\\dfrac{a}{4}(3\\sin \\theta-\\sin 3\\theta),"
"x'=\\dfrac{a}{4}(-3\\sin \\theta-3\\sin 3\\theta)"
"x''=\\dfrac{a}{4}(-3\\cos \\theta-9\\cos 3\\theta)"
"y'=\\dfrac{a}{4}(3\\cos \\theta-3\\cos 3\\theta)"
"y''=\\dfrac{a}{4}(-3\\sin \\theta+9\\sin 3\\theta)"
"R=\\dfrac{((x')^2+(y')^2)^{3\/2}}{|x'y''-x''y'|}"
"(x')^2+(y')^2"
"=\\dfrac{a^2}{16}(9\\sin^2 \\theta+9\\sin^2 3\\theta+18\\sin\\theta \\sin 3\\theta)"
"+\\dfrac{a^2}{16}(9\\cos^2 \\theta+9\\cos^2 3\\theta-18\\cos\\theta \\cos 3\\theta)"
"=\\dfrac{9a^2}{8}(1-\\cos4\\theta)=\\dfrac{9a^2}{4}\\sin^22\\theta"
"x'y''-x''y'"
"=\\dfrac{a^2}{16}(9\\sin^2 \\theta-27\\sin^2 3\\theta-18\\sin\\theta \\sin 3\\theta)"
"-\\dfrac{a^2}{16}(-9\\cos^2 \\theta+27\\cos^2 3\\theta-18\\cos\\theta \\cos 3\\theta)"
"=-\\dfrac{9a^2}{8}(1-\\cos4\\theta)=-\\dfrac{9a^2}{4}\\sin^22\\theta"
"R=\\dfrac{\\dfrac{27a^3}{8}\\sin^32\\theta}{|-\\dfrac{9a^2}{4}\\sin^22\\theta|}"
"=\\dfrac{3a}{2}\\sin2\\theta=3a\\sin\\theta \\cos\\theta"
"R=3a\\sin\\theta \\cos\\theta"
2.
"y=a(\\sin \\theta-\\theta \\cos \\theta)"
"x'=a(-\\sin \\theta+\\sin \\theta+\\theta\\cos \\theta)=a(\\theta \\cos \\theta)"
"x''=a(\\cos \\theta-\\theta \\sin \\theta )"
"y'=a(\\cos \\theta-\\cos \\theta+\\theta \\sin \\theta)=a(\\theta \\sin \\theta)"
"y''=a(\\sin \\theta+\\theta \\cos \\theta )"
"R=\\dfrac{((x')^2+(y')^2)^{3\/2}}{|x'y''-x''y'|}"
"(x')^2+(y')^2=a^2\\theta^2(\\cos62\\theta+\\sin^2\\theta)=a^2\\theta^2"
"((x')^2+(y')^2)^{3\/2}=a^3\\theta^3"
"x'y''-x''y'=a^2(\\theta\\sin\\theta \\cos\\theta+\\theta^2\\cos^2\\theta)"
"-a^2(\\theta\\sin\\theta \\cos\\theta-\\theta^2\\sin^2\\theta)=a^2\\theta^2"
"R=\\dfrac{a^3\\theta^3}{|a^2\\theta^2|}=a\\theta"
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