The parametric equation of a circle is given by \[x=3\cos \phi +2,\,y=3\sin \phi \]. Then its
\[\begin{align}
& \text{A}\text{.Centre}=\left( -2,0 \right) \\
& B.\text{Radius}=3 \\
& C.\text{Centre}=\left( 2,0 \right) \\
& D.\text{Radius}=1 \\
\end{align}\]
Answer
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Hint: As $x,y$ are in terms of \[\phi \]. Try to find values of $\sin \phi ,\cos \phi $ in terms of \[x,y\]. Now substitute them into the very known trigonometric identity. By this you get an equation of circle. From where you can use the formula of coordinate geometry. If a circle equation is given by \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c\] center is \[\left( -g,-f \right)\] and radius is \[\sqrt{{{g}^{2}}+{{f}^{2}}-c}\].
Complete step by step answer:
Given parametric form of the circle, can be written as:
\[\begin{align}
& x=3\cos \phi +2\ldots \ldots \ldots ..\text{ }\left( \text{1} \right) \\
& y=3\sin \phi \ldots \ldots \ldots \ldots \ldots \ldots \text{ }\left( \text{2} \right) \\
\end{align}\]
By taking equation (1) separately we can write it in the form:
\[x=3\cos \phi +2\]
By subtracting with 2 on both sides, we get it as of form,
\[x-2=3\cos \phi +2-2\]
By simplifying the above equation, we get it as:
\[x-2=3\cos \phi \]
By squaring on both sides of equation, we get it as:
\[{{\left( x-2 \right)}^{2}}={{\left( 3\cos \phi \right)}^{2}}\]
By simplifying the above equation, we get it as:
\[{{\left( x-2 \right)}^{2}}=9{{\cos }^{2}}\phi \ldots \ldots \ldots \ldots \ldots \text{ }\left( \text{3} \right)\]
By taking equation (2) separately, we get it in the form of:
\[y=3\sin \phi \]
By squaring on both sides of equation, we get it as:
\[{{y}^{2}}=9{{\sin }^{2}}\phi \ldots \ldots \ldots \ldots \ldots \left( \text{4} \right)\]
By basic general trigonometric knowledge, we can say the identity:
\[{{\cos }^{2}}\phi +{{\sin }^{2}}\phi =1\]
By multiplying with 9 on both sides of equation, we get it as:
\[9{{\cos }^{2}}\phi +9{{\sin }^{2}}\phi =9\]
By substituting equation (4) into above equation, we get it as:
\[{{\left( x-2 \right)}^{2}}+{{y}^{2}}=9\]
We know, \[{{\left( a-b \right)}^{2}}={{a}^{2}}+{{b}^{2}}-2ab\]
By expanding first term and subtracting 9 on both sides, we get:
\[{{x}^{2}}+4-4x+{{y}^{2}}-9=0\]
By simplifying the above equation, we get it in form:
\[{{x}^{2}}+{{y}^{2}}-4x-5=0\]
We know for a circle with equation \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\] center is \[\left( -g,-f \right)\] and radius is \[\sqrt{{{g}^{2}}+{{f}^{2}}-c}\].
By comparing both the equations, we get value of g, f, c as:
\[g=\dfrac{-4}{2}=-2\,,\,\,\,f=0\,\,,\,\,\,c=-5\]
By substituting it above, we get center, radius as:
\[\text{Center}=\left( -\left( -2 \right),0 \right)\,\,;\,\,\,\text{Radius}\,=\sqrt{{{2}^{2}}-\left( -5 \right)}=\sqrt{4+5}\]
By simplifying we can say their values to be as:
\[\text{Center}\,=\,\left( 2,0 \right)\,\,;\,\,\text{Radius}\,=3\]
So, the correct answers are “Option B and C”.
Note: Be careful while taking the parametric form. The idea of leaving equation at stage of \[9{{\cos }^{2}}\phi \,,\,\,9{{\sin }^{2}}\phi \] is the idea to avoid fractions. It is very important that you stop there or else it will become long to solve. But try to minimize as much as possible.
Complete step by step answer:
Given parametric form of the circle, can be written as:
\[\begin{align}
& x=3\cos \phi +2\ldots \ldots \ldots ..\text{ }\left( \text{1} \right) \\
& y=3\sin \phi \ldots \ldots \ldots \ldots \ldots \ldots \text{ }\left( \text{2} \right) \\
\end{align}\]
By taking equation (1) separately we can write it in the form:
\[x=3\cos \phi +2\]
By subtracting with 2 on both sides, we get it as of form,
\[x-2=3\cos \phi +2-2\]
By simplifying the above equation, we get it as:
\[x-2=3\cos \phi \]
By squaring on both sides of equation, we get it as:
\[{{\left( x-2 \right)}^{2}}={{\left( 3\cos \phi \right)}^{2}}\]
By simplifying the above equation, we get it as:
\[{{\left( x-2 \right)}^{2}}=9{{\cos }^{2}}\phi \ldots \ldots \ldots \ldots \ldots \text{ }\left( \text{3} \right)\]
By taking equation (2) separately, we get it in the form of:
\[y=3\sin \phi \]
By squaring on both sides of equation, we get it as:
\[{{y}^{2}}=9{{\sin }^{2}}\phi \ldots \ldots \ldots \ldots \ldots \left( \text{4} \right)\]
By basic general trigonometric knowledge, we can say the identity:
\[{{\cos }^{2}}\phi +{{\sin }^{2}}\phi =1\]
By multiplying with 9 on both sides of equation, we get it as:
\[9{{\cos }^{2}}\phi +9{{\sin }^{2}}\phi =9\]
By substituting equation (4) into above equation, we get it as:
\[{{\left( x-2 \right)}^{2}}+{{y}^{2}}=9\]
We know, \[{{\left( a-b \right)}^{2}}={{a}^{2}}+{{b}^{2}}-2ab\]
By expanding first term and subtracting 9 on both sides, we get:
\[{{x}^{2}}+4-4x+{{y}^{2}}-9=0\]
By simplifying the above equation, we get it in form:
\[{{x}^{2}}+{{y}^{2}}-4x-5=0\]
We know for a circle with equation \[{{x}^{2}}+{{y}^{2}}+2gx+2fy+c=0\] center is \[\left( -g,-f \right)\] and radius is \[\sqrt{{{g}^{2}}+{{f}^{2}}-c}\].
By comparing both the equations, we get value of g, f, c as:
\[g=\dfrac{-4}{2}=-2\,,\,\,\,f=0\,\,,\,\,\,c=-5\]
By substituting it above, we get center, radius as:
\[\text{Center}=\left( -\left( -2 \right),0 \right)\,\,;\,\,\,\text{Radius}\,=\sqrt{{{2}^{2}}-\left( -5 \right)}=\sqrt{4+5}\]
By simplifying we can say their values to be as:
\[\text{Center}\,=\,\left( 2,0 \right)\,\,;\,\,\text{Radius}\,=3\]
So, the correct answers are “Option B and C”.
Note: Be careful while taking the parametric form. The idea of leaving equation at stage of \[9{{\cos }^{2}}\phi \,,\,\,9{{\sin }^{2}}\phi \] is the idea to avoid fractions. It is very important that you stop there or else it will become long to solve. But try to minimize as much as possible.
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