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Let the ellipse $$3x^2 + py^2 = 4$$ pass through the centre C of the circle $$x^2 + y^2 - 2x - 4y - 11 = 0$$ of radius r. Let $$f_1$$, $$f_2$$ be the focal distances of the point C on the ellipse. Then $$6f_1f_2 - r$$ is equal to
The centre of the circle $$x^{2}+y^{2}-2x-4y-11=0$$ is obtained by completing the squares.
$$x^{2}-2x+1+y^{2}-4y+4=11+1+4$$
$$\Rightarrow (x-1)^{2}+(y-2)^{2}=16$$
Hence the centre is $$C(1,2)$$ and the radius is $$r=4$$.
The ellipse is $$3x^{2}+py^{2}=4$$ and it passes through the point $$C(1,2)$$, so
$$3(1)^{2}+p(2)^{2}=4$$
$$\Rightarrow 3+4p=4$$
$$\Rightarrow 4p=1 \;\Longrightarrow\; p=\frac14$$
Therefore the ellipse is $$3x^{2}+\frac14\,y^{2}=4$$.
Divide by $$4$$ to get the standard form:
$$\frac{3}{4}x^{2}+\frac{1}{16}y^{2}=1$$
$$\Rightarrow \frac{x^{2}}{\frac{4}{3}}+\frac{y^{2}}{16}=1$$
Comparing with $$\frac{x^{2}}{b^{2}}+\frac{y^{2}}{a^{2}}=1$$ (major axis along $$y$$-axis), we have
$$a^{2}=16,\;a=4,\qquad b^{2}=\frac{4}{3}$$
The focal length satisfies $$c^{2}=a^{2}-b^{2}$$, hence
$$c^{2}=16-\frac{4}{3}=\frac{48-4}{3}=\frac{44}{3},\qquad c=\sqrt{\frac{44}{3}}$$
The foci are $$F_{1}(0,c)$$ and $$F_{2}(0,-c)$$.
Let the focal distances of the point $$C(1,2)$$ be $$f_{1}=CF_{1}$$ and $$f_{2}=CF_{2}$$.
Since $$C$$ lies on the ellipse, the sum of its focal distances equals the major axis length:
$$f_{1}+f_{2}=2a=8 \quad -(1)$$
Now compute the squares of the distances.
$$f_{1}^{2}=(1-0)^{2}+(2-c)^{2}=1+(2-c)^{2}
=1+4-4c+c^{2}=5-4c+c^{2}$$
$$f_{2}^{2}=(1-0)^{2}+(2+c)^{2}=1+(2+c)^{2}
=1+4+4c+c^{2}=5+4c+c^{2}$$
Adding:
$$f_{1}^{2}+f_{2}^{2}=(5-4c+c^{2})+(5+4c+c^{2}) =10+2c^{2} \quad -(2)$$
From identities,
$$(f_{1}+f_{2})^{2}=f_{1}^{2}+f_{2}^{2}+2f_{1}f_{2} \quad -(3)$$
Substitute $$-(1)$$ and $$-(2)$$ into $$-(3)$$:
$$8^{2}=10+2c^{2}+2f_{1}f_{2}$$
$$64=10+2\left(\frac{44}{3}\right)+2f_{1}f_{2}$$
$$64=10+\frac{88}{3}+2f_{1}f_{2}$$
$$64=\frac{30}{3}+\frac{88}{3}+2f_{1}f_{2}
=\frac{118}{3}+2f_{1}f_{2}$$
$$\Rightarrow 2f_{1}f_{2}=64-\frac{118}{3} =\frac{192-118}{3}=\frac{74}{3}$$
$$\therefore\; f_{1}f_{2}=\frac{37}{3}$$
Finally,
$$6f_{1}f_{2}-r=6\left(\frac{37}{3}\right)-4 =2\cdot37-4 =74-4 =70$$
Hence $$6f_{1}f_{2}-r=70$$, which matches Option C.
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