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NCERT Exemplar · Q59

Q.The locus of the point of intersection of lines 3x−y−43k=0\sqrt{3}x - y - 4\sqrt{3}k = 0 and 3kx+ky−43=0\sqrt{3}kx + ky - 4\sqrt{3} = 0 for different value of kk is a hyperbola whose eccentricity is 2.

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Eliminating kk from the two lines gives the locus 3x2−y2=483x^2 - y^2 = 48, i.e. the hyperbola x216−y248=1\dfrac{x^2}{16} - \dfrac{y^2}{48} = 1, whose eccentricity is 1+4816=2\sqrt{1+\tfrac{48}{16}} = 2. The statement is TRUE.

The two given lines each depend on a parameter kk; for each value of kk they meet at a point (x,y)(x, y). As kk varies, that intersection point moves and traces a curve — the locus. To find it we eliminate kk between the two equations, leaving a relation in xx and yy only.

The lines are

L1: 3 x−y−43 k=0,L_1:\ \sqrt{3}\,x - y - 4\sqrt{3}\,k = 0,

L2: 3 k x+k y−43=0.L_2:\ \sqrt{3}\,k\,x + k\,y - 4\sqrt{3} = 0.

Step 1 — Isolate the kk-dependence in each line

From L1L_1:

3 x−y=43 k.(1)\sqrt{3}\,x - y = 4\sqrt{3}\,k. \qquad(1)

From L2L_2, factor kk out of the first two terms:

k(3 x+y)=43  ⇒  3 x+y=43k.(2)k\left(\sqrt{3}\,x + y\right) = 4\sqrt{3} \;\Rightarrow\; \sqrt{3}\,x + y = \frac{4\sqrt{3}}{k}. \qquad(2)

Step 2 — Multiply the two relations to cancel kk

Multiplying (1) by (2), the kk on the right cancels:

(3 x−y)(3 x+y)=43 k⋅43k=16⋅3=48.\left(\sqrt{3}\,x - y\right)\left(\sqrt{3}\,x + y\right) = 4\sqrt{3}\,k \cdot \frac{4\sqrt{3}}{k} = 16 \cdot 3 = 48.

The left side is a difference of squares:

(3 x)2−y2=48  ⇒  3x2−y2=48.(\sqrt{3}\,x)^2 - y^2 = 48 \;\Rightarrow\; 3x^2 - y^2 = 48.

Step 3 — Put the locus in standard hyperbola form

Divide through by 4848:

3x248−y248=1  ⇒  x216−y248=1.\frac{3x^2}{48} - \frac{y^2}{48} = 1 \;\Rightarrow\; \frac{x^2}{16} - \frac{y^2}{48} = 1.

This is a hyperbola x2a2−y2b2=1\dfrac{x^2}{a^2} - \dfrac{y^2}{b^2} = 1 with

a2=16,b2=48.a^2 = 16, \qquad b^2 = 48.

Step 4 — Compute the eccentricity …

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