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A circle has a radius of 17 cm. A chord AB is drawn such that the distance from the centre O to the chord is 8 cm. What is the length of AB?
15 cm
30 cm
25 cm
34 cm
30 cm
Recognize the Pythagorean triple (8, 15, 17). Since the radius is 17 and distance is 8, the half-chord length must be 15. The full chord is 2 * 15 = 30.
Radius of the circle R = 17 cm, distance from center O to chord AB d = 8 cm.
r2=d2+(2LтАЛ)2
Recognize the Pythagorean triple (8, 15, 17). Since the radius is 17 and distance is 8, the half-chord length must be 15. The full chord is 2 * 15 = 30.
Students often forget to double the value of the half-chord obtained from the Pythagorean theorem, incorrectly selecting 15 cm.
Setup the geometric relationship
Draw a perpendicular from the center to the chord, which bisects the chord. This forms a right-angled triangle with the radius as the hypotenuse, the distance as one leg, and half the chord as the other leg.
R2=d2+x2
Substitute the given values
Substitute R=17 and d=8 into the equation to find the half-chord length x.
172=82+x2тЯ╣289=64+x2
Solve for half-chord length
Isolate x2 and calculate the square root to find the length of the segment.
x2=289тИТ64=225тЯ╣x=225тАЛ=15
Calculate the full chord length
Since the perpendicular from the center bisects the chord, the total length of the chord AB is 2x.
AB=2├Ч15=30
B is correct because the half-chord length is calculated as 15 cm using the Pythagorean theorem, making the total chord length 30 cm.
This concept of Pythagorean triples is frequently used in Coordinate Geometry to find the length of tangents and intersecting chords in circles.