arthham

JEE Advanced 2016 · Conic Sections

Let PP be the point on the parabola y2=4xy^{2} = 4x which is at the shortest distance from the center SS of the circle x2+y2−4x−16y+64=0x^{2} + y^{2} - 4x - 16y + 64 = 0. Let QQ be the point on the circle dividing the line segment SPSP internally. Then (1) SP=25SP = 2\sqrt{5} (2) SQ:QP=(5+1):2SQ : QP = \left(\sqrt{5} + 1\right) : 2 (3) the xx-intercept of the normal to the parabola at PP is 66 (4) the slope of the tangent to the circle at QQ is 12\frac{1}{2}

Answers: A. SP=25SP = 2\sqrt{5} · C. the xx-intercept of the normal to the parabola at PP is 66 · D. the slope of the tangent to the circle at QQ is 12\frac{1}{2}

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Written solution

Let P be the point on the parabola y² = 4x which is at the shortest distance from the center S of the circle x² + y² - 4x - 16y + 64 = 0. Let Q be the point on the circle dividing the line segment SP internally. Then

y2=4x(parabola)y^2 = 4x \quad (\text{parabola})

x2+y2−4x−16y+64=0(circle)x^2+y^2-4x-16y+64=0 \quad (\text{circle})

S=centre of the circleS = \text{centre of the circle}

P∈y2=4x nearest to SP \in y^2=4x \text{ nearest to } S

Q∈circle, Q divides SP internallyQ \in \text{circle}, \ Q \text{ divides } SP \text{ internally}

To find: correct statement(s)\text{To find: correct statement(s)}

x2+y2−4x−16y+64=0x^2+y^2-4x-16y+64=0 ⇒(x−2)2+(y−8)2=4\Rightarrow (x-2)^2+(y-8)^2=4 ⇒S(2,8),r=2\Rightarrow S(2,8),\quad r=2 y2=4x⇒a=1, vertex (0,0)y^2=4x \Rightarrow a=1,\ \text{vertex } (0,0) 1. shortest SP⇒SP is the normal at P1.\ \text{shortest } SP \Rightarrow SP \text{ is the normal at } P 2. SP through S⇒P2.\ SP \text{ through } S \Rightarrow P 3. check options (1)–(4)3.\ \text{check options (1)--(4)} SP⊥tangent at P⇒SP is the normalSP \perp \text{tangent at } P \Rightarrow SP \text{ is the normal} normal at param t: y=−t x+2t+t3 (a=1)\text{normal at param } t:\ y = -t\,x + 2t + t^3 \ (a=1) S(2,8): 8=−2t+2t+t3⇒t3=8S(2,8):\ 8 = -2t + 2t + t^3 \Rightarrow t^3 = 8 ⇒t=2\Rightarrow t = 2 P=(t2,2t)=(4,4)P = (t^2, 2t) = (4,4) SP=(4−2)2+(4−8)2=20=25 ✓SP = \sqrt{(4-2)^2+(4-8)^2} = \sqrt{20} = 2\sqrt{5} \ \checkmark SQ=r=2,QP=25−2SQ = r = 2,\quad QP = 2\sqrt{5}-2 SQQP=225−2=15−1=5+14≠5+12 ×\dfrac{SQ}{QP} = \dfrac{2}{2\sqrt5-2} = \dfrac{1}{\sqrt5-1} = \dfrac{\sqrt5+1}{4} \neq \dfrac{\sqrt5+1}{2} \ \times t=2: y=−2x+12t=2:\ y = -2x+12 y=0⇒x=6 ✓y=0 \Rightarrow x = 6 \ \checkmark mSQ=4−84−2=−2m_{SQ} = \dfrac{4-8}{4-2} = -2 mtangent=−1−2=12 ✓m_{\text{tangent}} = -\dfrac{1}{-2} = \dfrac12 \ \checkmark True: (1), (3), (4);(2) is false\text{True: (1), (3), (4)};\quad (2) \text{ is false} Correct options: (1), (3), (4)\boxed{\text{Correct options: (1), (3), (4)}}

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