4 The diagram shows a sector \(A O B\) of a circle with centre \(O\) and radius \(r \mathrm {~cm}\).
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The angle \(A O B\) is \(\theta\) radians. The arc length \(A B\) is 15 cm and the area of the sector is \(45 \mathrm {~cm} ^ { 2 }\).
- Find the values of \(r\) and \(\theta\).
- Find the area of the segment bounded by the arc \(A B\) and the chord \(A B\).