Equation of Locus of a Point Examples :
Here we are going to see how to find equation of locus of a point with the given condition.
Question 1 :
If O is origin and R is a variable point on y2 = 4x, then find the equation of the locus of the mid-point of the line segment OR.
Solution :
Let the coordinates R be (a, b) and let P (h, k) be the midpoint of OQ. Then
h = (a + 0)/2 = a/2
k = (0 + b)/2 = b/2
a = 2h and b = 2k
Here a and b are two variables which are to be eliminated. Since (a, b) lies on y2 = 4x,
b2 = 4a
(2k)2 = 4 (2h)
4k2 = 8h
h = x and k = y
4y2 = 8x
y2 = 2x
Hence the equation of locus y2 = 2x.
Question 2 :
The coordinates of a moving point P are (a/2 (cosec θ + sin θ) , b/2 (cosecθ − sin θ)), where θ is a variable parameter. Show that the equation of the locus P is b2x2 − a2y2 = a2b2 .
Solution :
h = a/2 (cosec θ + sin θ)
cosec θ + sin θ = 2h/a
(cosec θ + sin θ)2 = (2h/a)2
cosec2 θ + sin2θ + 2cosecθ sin θ = 4h2/a2
cosec2 θ + sin2θ + 2(1/sin θ) sin θ = 4h2/a2
cosec2 θ + sin2θ + 2 = 4h2/a2 ------(1)
k = b/2 (cosec θ - sin θ)
cosec θ - sin θ = 2k/b
(cosec θ - sin θ)2 = (2k/b)2
cosec2 θ + sin2θ - 2cosecθ sin θ = 4k2/b2
cosec2 θ + sin2θ - 2(1/sin θ) sin θ = 4k2/b2
cosec2 θ + sin2θ - 2 = 4k2/b2 ------(2)
(1) - (2)
(cosec2θ+sin2θ+2)-(cosec2θ+sin2θ-2) = (4h2/a2) - (4k2/b2)
4 = (4h2/a2) - (4k2/b2)
Divide by 4 on both sides.
1 = (h2/a2) - (k2/b2)
a2b2 = h2b2 - k2a2
Replacing h and k by x and y respectively.
a2b2 = x2b2 - y2a2
Question 3 :
If P(2,−7) is a given point and Q is a point on 2x2 + 9y2 = 18, then find the equations of the locus of the mid-point of PQ.
Solution :
Let Q be (a, b)
Midpoint of the line segment PQ :
Let A be the midpoint of the line segment PQ (h, k)
P (2, - 7) and Q (a, b)
Midpoint of PQ = (x1 + x2) /2, (x1 + x2) /2
(h , k) = (2 + a)/2, (-7 + b)/2
(2 + a) /2 = h 2 + a = 2 h a = 2h - 2 |
(-7 + b) /2 = k -7 + b = 2 k b = 2k + 7 |
Since the point Q lies on the curve
2x2 + 9y2 = 18
2a2 + 9b2 = 18 -----(1)
By applying the values of a and b in the above equation, we get
2(2h - 2)2 + 9(2k + 7)2 = 18
2(4h2 - 8h + 4) + 9(4k2 + 28k + 49) = 18
8h2 - 16h + 8 + 36k2 + 72k + 441 = 18
8h2 + 36k2- 16h + 72k + 449 - 18 = 0
8h2 + 36k2- 16h + 72k + 431 = 0
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