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JEE Advanced 2007 · Straight Lines

Lines L1:y−x=0L_1 : y - x = 0 and L2:2x+y=0L_2 : 2x + y = 0 intersect the line L3:y+2=0L_3 : y + 2 = 0 at PP and QQ, respectively. The bisector of the acute angle between L1L_1 and L2L_2 intersects L3L_3 at RR. STATEMENT-I : The ratio PR:RQPR : RQ equals 22:52\sqrt{2} : \sqrt{5}. STATEMENT-II : In any triangle, bisector of an angle divides the triangle into two similar triangles.

Answer: C. Statement-I is True, Statement-II is False

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Written solution

Lines L₁ : y - x = 0 and L₂ : 2x + y = 0 intersect the line L₃ : y + 2 = 0 at P and Q, respectively. The bisector of the acute angle between L₁ and L₂ intersects L₃ at R. STATEMENT-I : The ratio PR : RQ equals 2√{2} : √{5}. STATEMENT-II : In any triangle, bisector of an angle divides the triangle into two similar triangles.

Given: L1:y−x=0,L2:2x+y=0,L3:y+2=0\text{Given: } L_1 : y - x = 0,\quad L_2 : 2x + y = 0,\quad L_3 : y + 2 = 0

P=L1∩L3,Q=L2∩L3P = L_1 \cap L_3,\qquad Q = L_2 \cap L_3

To check: (I) PR:RQ=22:5\text{To check: (I) } PR : RQ = 2\sqrt{2} : \sqrt{5}

(II) angle bisector in a triangle gives two similar triangles\text{(II) angle bisector in a triangle gives two similar triangles}

L1:y=xL_1 : y = x

L2:y=−2xL_2 : y = -2x

L3:y+2=0 ⇒ y=−2L_3 : y + 2 = 0 \ \Rightarrow \ y = -2

L1∩L3: x=−2 ⇒ P(−2,−2)L_1 \cap L_3:\ x = -2 \ \Rightarrow\ P(-2,-2)

L2∩L3: −2x=−2 ⇒ x=1 ⇒ Q(1,−2)L_2 \cap L_3:\ -2x = -2 \ \Rightarrow\ x = 1 \ \Rightarrow\ Q(1,-2)

OP⊂L1,OQ⊂L2OP \subset L_1,\quad OQ \subset L_2

⇒ ∠ POQ = acute angle between L₁ and L₂

  1. angle-bisector theorem in triangle OPQ

2. find the lengths OP and OQ2.\ \text{find the lengths } OP \text{ and } OQ

3. check Statement-I, then Statement-II3.\ \text{check Statement-I, then Statement-II}

PRRQ=OPOQ(angle-bisector theorem)\frac{PR}{RQ} = \frac{OP}{OQ}\quad(\text{angle-bisector theorem})

OP=(−2)2+(−2)2=8=22OP = \sqrt{(-2)^2 + (-2)^2} = \sqrt{8} = 2\sqrt{2}

OQ=12+(−2)2=5OQ = \sqrt{1^2 + (-2)^2} = \sqrt{5}

PRRQ=225\frac{PR}{RQ} = \frac{2\sqrt{2}}{\sqrt{5}}

⇒ PR:RQ=22:5 ⇒ Statement-I is TRUE\Rightarrow\ PR : RQ = 2\sqrt{2} : \sqrt{5}\ \Rightarrow\ \text{Statement-I is TRUE}

II claims triangle OPR ∼ triangle OQR

but the theorem only gives PRRQ=OPOQ\text{but the theorem only gives } \frac{PR}{RQ} = \frac{OP}{OQ}

∠ ORP + ∠ ORQ = 180^∘ (P, R, Q collinear) similar ⇒ ∠ ORP = ∠ ORQ = 90^∘ ⇒ OR ⊥ PQ

here OR is not perpendicular to PQ\text{here } OR \text{ is not perpendicular to } PQ

⇒ Statement-II is FALSE\Rightarrow\ \text{Statement-II is FALSE}

Statement-I True, Statement-II False ⇒ (3)\boxed{\text{Statement-I True, Statement-II False} \ \Rightarrow\ (3)}

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