1.
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\caption{Figure 1}
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Figure 1 shows a sector \(A O B\) of a circle with centre \(O\) and radius \(r \mathrm {~cm}\).
The angle \(A O B\) is 1.25 radians.
Given that the area of the sector \(A O B\) is \(15 \mathrm {~cm} ^ { 2 }\)
- find the exact value of \(r\),
- find the exact length of the perimeter of the sector. Write your answer in simplest form.