Question 39 of 50intermediate🔧 ApplyDiagram4 marks

Draw a circle in which a chord of 6 cm length stands at a distance of 3 cm from the centre. (Hint: Is it a circumcircle of a suitable triangle?)

Correct Answer

To draw the required circle, we first calculate its radius using the relation chord length = 2r2−d22\sqrt{r^2 - d^2}. Given chord length is 6 cm and perpendicular distance dd is 3 cm, we have 6=2r2−326 = 2\sqrt{r^2 - 3^2}. Solving this, 3=r2−93 = \sqrt{r^2 - 9}, so 9=r2−99 = r^2 - 9, which gives r2=18r^2 = 18 and radius r=32r = 3\sqrt{2} cm.

Following the hint, we construct the circle as the circumcircle of a suitable triangle. A right-angled isosceles triangle with legs of 6 cm each is suitable. First, draw △ABC\triangle ABC with ∠B=90∘\angle B = 90^\circ and AB = BC = 6 cm. Next, find the midpoint O of the hypotenuse AC. Since the midpoint of the hypotenuse is the circumcentre for a right triangle, O is the centre. Draw the circle with centre O and radius OA (323\sqrt{2} cm).

In this circle, AB is a chord of 6 cm. The perpendicular distance from the centre O to the chord AB is half the length of the other leg BC (as O is the midpoint of AC). Thus, the distance is 6/2=36 / 2 = 3 cm, satisfying the given conditions.

Exercise: END-OF-CHAPTER EXERCISES | Q: *13 | (Page 24)
For More Understanding

Explanation

The student uses the formula for chord length provided in the context to find the radius. The hint suggests using a circumcircle of a suitable triangle. A right-angled isosceles triangle with legs equal to the chord length (6 cm) has its circumradius equal to half the hypotenuse, which is 323\sqrt{2} cm. The distance from the circumcentre (midpoint of hypotenuse) to either leg is exactly half the other leg, which is 3 cm. This elegantly satisfies the problem's conditions.

Solution Steps

  1. Step 1: Calculate the radius rr using the formula 6=2r2−326 = 2\sqrt{r^2 - 3^2}, yielding r=32r = 3\sqrt{2} cm.

  2. Step 2: Draw a right-angled isosceles triangle ABC with ∠B=90∘\angle B = 90^\circ and AB = BC = 6 cm.

  3. Step 3: Find the midpoint O of the hypotenuse AC.

  4. Step 4: Draw the circle with centre O and radius OA = 323\sqrt{2} cm. Chord AB is 6 cm and its distance from O is 3 cm.