JEE MAIN - Mathematics (2022 - 24th June Evening Shift - No. 24)

Let P1 be a parabola with vertex (3, 2) and focus (4, 4) and P2 be its mirror image with respect to the line x + 2y = 6. Then the directrix of P2 is x + 2y = ____________.
Answer
10

Explanation

Focus = (4, 4) and vertex = (3, 2)

$$\therefore$$ Point of intersection of directrix with axis of parabola = A = (2, 0)

Image of A(2, 0) with respect to line x + 2y = 6 is B(x2, y2)

$$\therefore$$ $${{{x_2} - 2} \over 1} = {{{y_2} - 0} \over 2} = {{ - 2(2 + 0 - 6)} \over 5}$$

$$\therefore$$ $$B({x_2},\,{y_2}) = \left( {{{18} \over 5},{{16} \over 5}} \right)$$.

Point B is point of intersection of direction with axes of parabola P2.

$$\therefore$$ $$x + 2y = \lambda $$ must have point $$\left( {{{18} \over 5},{{16} \over 5}} \right)$$

$$\therefore$$ $$x + 2y = 10$$

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