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Two circles have arcs of equal length that subtend angles of 60 deg and 75 deg respectively at their centres. Find the ratio of the radii of the two circles.

1. 5: 4
2. 4: 5
3. 5: 6
4. 3: 4

Correct answer: 5: 4

Solution

Equal arc lengths give r1*theta1 = r2*theta2, so r1:r2 = theta2:theta1 = 75:60 = 5:4.

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