1.

Let L be the point (t, 2) and M be a point on the y-axis such that the slope of LM is −t. Then, the locus of the midpoint of LM is a parabola whose latus rectum is(a)  2(b)   1/2(c)   4(d)  1/4

Answer»

Correct option (b)  1/2

Explanation :

Let M = (0, k) so that the slope of LM is

2 - k/t - 0 = -t

⇒ 2 - k = -t2   ....(1)

Let (x, y) be the midpoint of LM. Therefore

x = t/2 and y = 2 + k/2

Hence, from Eq. (1), we have

2y = 2 + k = 2 + (2 + t2)

= 4 + t2

= 4 + 4x2

y = 2 + 2x2

x2 = 1/2(y - 2)

Hence, the latus rectum is 1/2.



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