Question 38 of 50advanced🔧 ApplyLong Answer5 marks

When two chords intersect, each of them is divided into two line segments. Show that if the intersecting chords are of equal length, then the line segments of one chord are equal to the corresponding line segments of the other chord.

Correct Answer

Let the two equal chords be ABAB and CDCD, intersecting at point EE. Let OO be the centre of the circle. We draw perpendiculars OMOM and ONON from OO to ABAB and CDCD respectively. We are given that AB=CDAB = CD.

Since a perpendicular from the centre to a chord bisects it, MM is the midpoint of ABAB and NN is the midpoint of CDCD. Thus,

AM=MB=12ABAM = MB = \frac{1}{2}AB

and

CN=ND=12CDCN = ND = \frac{1}{2}CD

Because AB=CDAB = CD, we get AM=CNAM = CN and MB=NDMB = ND. Also, chords of equal length lie at equal distance from the centre, so OM=ONOM = ON.

Now, consider the right triangles △OME\triangle OME and △ONE\triangle ONE. We have OM=ONOM = ON, OE=OEOE = OE (common side), and ∠OME=∠ONE=90°\angle OME = \angle ONE = 90°. By the RHS congruence rule, △OME≅△ONE\triangle OME \cong \triangle ONE. Hence, their corresponding parts are equal, giving ME=NEME = NE.

Since MM and NN are midpoints, the segments of chord ABAB are:

AE=AM+MEAE = AM + ME

and

EB=MB−MEEB = MB - ME

(assuming EE is between MM and BB). Similarly, for chord CDCD:

CE=CN+NECE = CN + NE

and

ED=ND−NEED = ND - NE

Substituting AM=CNAM = CN and ME=NEME = NE, we get AE=CEAE = CE. Similarly, substituting MB=NDMB = ND and ME=NEME = NE, we get EB=EDEB = ED.

∴ The line segments of one chord are equal to the corresponding line segments of the other chord.

Exercise: END-OF-CHAPTER EXERCISES | Q: *12 | (Page 24)
For More Understanding

Explanation

The proof leverages two key properties provided in the textbook context: first, that a perpendicular from the centre bisects a chord, and second, that equal chords are equidistant from the centre. By dropping perpendiculars from the centre to the two equal intersecting chords, we establish that the halves of the chords are equal, and the perpendicular distances are equal.

This sets up the conditions for the RHS congruency of the right triangles formed by the centre, the midpoint, and the point of intersection. Proving these triangles congruent shows that the distances from the intersection point to the midpoints are equal, which ultimately allows us to algebraically prove that the corresponding segments of the intersecting chords are equal.

Solution Steps

  1. Step 1: Let equal chords ABAB and CDCD intersect at EE, with OO as the centre. Draw perpendiculars OMOM to ABAB and ONON to CDCD.

  2. Step 2: Apply the property that a perpendicular from the centre bisects the chord, so AM=MB=12ABAM = MB = \frac{1}{2}AB and CN=ND=12CDCN = ND = \frac{1}{2}CD. Since AB=CDAB = CD, AM=CNAM = CN and MB=NDMB = ND.

  3. Step 3: Apply the property that equal chords are equidistant from the centre, so OM=ONOM = ON.

  4. Step 4: In right triangles △OME\triangle OME and △ONE\triangle ONE, OM=ONOM = ON, OE=OEOE = OE, and ∠OME=∠ONE=90°\angle OME = \angle ONE = 90°. By RHS congruence, △OME≅△ONE\triangle OME \cong \triangle ONE, meaning ME=NEME = NE.

  5. Step 5: Add/subtract the equal segments (AE=AM+MEAE = AM + ME, CE=CN+NECE = CN + NE, etc.) to conclude AE=CEAE = CE and EB=EDEB = ED.