Q. 115.0( 1 Vote )

Find the locus of the mid-point of the portion of the x cos α + y sin α = p which is intercepted between the axes.

Answer :

Key points to solve the problem:


Idea of distance formula- Distance between two points A(x1,y1) and B(x2,y2) is given by- AB =


Idea of section formula- Let two points A(x1,y1) and B(x2,y2) forms a line segment. If a point C(x,y) divides line segment AB in the ratio of m:n internally, then coordinates of C is given as:


C = when m = n =1 , C becomes the midpoint of AB and C is given as C =


How to approach: To find the locus of a point we first assume the coordinate of the point to be (h, k) and write a mathematical equation as per the conditions mentioned in the question and finally replace (h, k) with (x, y) to get the locus of the point.


Let the coordinates of a point whose locus is to be determined to be (h, k). Name the moving point to be C


Given that (h,k) is the midpoint of line x cos α + y sin α = p intercepted between axes.


So we need to find the points at which x cos α + y sin α = p cuts the axes after which we will apply the section formula to get the locus.


Put y = 0


x = p/cos α coordinates on x-axis is (p/cos α , 0). Name the point A


Similarly, Put x = 0


y = p/sin α coordinates on y-axis is (0, p/sin α ). Name this point B


As C(h,k) is the midpoint of AB


coordinate of C is given by:


C =


Thus,


…equation 1


and …equation 2


Squaring and adding equation 1 and 2:




Replace (h,k) with (x,y)


Thus, the locus of a point on the rod is: ….ans


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