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      Question

      Two circles of radius 13 cm and 15 cm intersect each

      other at points A and B. If the length of the common chord is 12 cm, then what is the distance between their centres?
      A √145 + √184 Correct Answer Incorrect Answer
      B √131 + √181 Correct Answer Incorrect Answer
      C √145 + √169 Correct Answer Incorrect Answer
      D √133 + √189 Correct Answer Incorrect Answer

      Solution

      Let O and R be the center of the circle and PQ be the length of the common chord. OP = 15, RP = 13 PS = 1/2 PQ (as perpendicular from the center of the circle to a chord bisects the chord) PS = 1/2 Γ— 12 PS = 6 By applying Pythagoras theorem in PSO: OS2 = OP2-PS2 = 152 – 62 = 225-36 = 189 Β  OS = √189 Β  By applying Pythagoras' theorem in PSO': RSΒ² = RP2-PS2 = 132 - 62 = 169-36 = 133 RS = √133 OR = OS + RS = √189 + √133 The distance between their centers = √189 + √133

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