arthham

JEE Advanced 2019 · Conic Sections

A line y=mx+1y = mx + 1 intersects the circle (x−3)2+(y+2)2=25(x - 3)^{2} + (y + 2)^{2} = 25 at the points P and Q. If the midpoint of the line segment PQ has x-coordinate −35-\frac{3}{5}, then which one of the following options is correct ?

Answer: A. 2≤m<42 \le m < 4

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Written solution

A line y = mx + 1 intersects the circle (x - 3)² + (y + 2)² = 25 at the points P and Q. If the midpoint of the line segment PQ has x-coordinate -3/5, then which one of the following options is correct ?

Given: y=mx+1 cuts (x−3)2+(y+2)2=25 at P, Q\text{Given: } y = mx + 1 \text{ cuts } (x-3)^2 + (y+2)^2 = 25 \text{ at } P,\ Q

Midpoint of PQ has x=−35\text{Midpoint of } PQ \text{ has } x = -\dfrac{3}{5}

To find: which range of m is correct\text{To find: which range of } m \text{ is correct}

Circle:(x−3)2+(y+2)2=25⇒C(3,−2), r=5\text{Circle}: (x-3)^2 + (y+2)^2 = 25 \Rightarrow C(3,-2),\ r = 5

y=mx+1 always passes through A(0,1)y = mx + 1 \text{ always passes through } A(0, 1)

M=foot of perpendicular from C onto the lineM = \text{foot of perpendicular from } C \text{ onto the line}

Line: mx−y+1=0 (a=m, b=−1, c=1)\text{Line: } mx - y + 1 = 0 \ (a = m,\ b = -1,\ c = 1)

xM=xC−a⋅axC+byC+ca2+b2x_M = x_C - a\cdot\dfrac{a x_C + b y_C + c}{a^2 + b^2}

xM=3−m(3m+3)m2+1x_M = 3 - \dfrac{m(3m + 3)}{m^2 + 1}

3−m(3m+3)m2+1=−353 - \dfrac{m(3m+3)}{m^2+1} = -\dfrac{3}{5}

⇒m(3m+3)m2+1=185\Rightarrow \dfrac{m(3m+3)}{m^2+1} = \dfrac{18}{5}

5(3m2+3m)=18(m2+1)5(3m^2 + 3m) = 18(m^2 + 1)

3m2−15m+18=03m^2 - 15m + 18 = 0

m2−5m+6=0m^2 - 5m + 6 = 0

(m−2)(m−3)=0⇒m=2 or m=3(m-2)(m-3) = 0 \Rightarrow m = 2 \text{ or } m = 3

d=∣3m+3∣m2+1d = \dfrac{|3m + 3|}{\sqrt{m^2 + 1}}

m=2: 95<5,m=3: 1210<5m = 2:\ \dfrac{9}{\sqrt5} < 5, \qquad m = 3:\ \dfrac{12}{\sqrt{10}} < 5

∠ CMA = 90^∘ ⇒ M lies on the circle with diameter CA

m∈{2, 3}⊂[2, 4)m \in \{2,\ 3\} \subset [2,\ 4)

option (1): 2≤m<4\boxed{\text{option (1): } 2 \le m < 4}

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