Question

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Answer 1

The angle of the shaded sector is a right angle, so it measures :
\[\theta=\frac\pi2\text{ rd}\]
The area is :
\[A=\frac\theta2\times r^2\]
where r is the radius !

Answer 2

The area is :
\[\frac{\frac\pi2}{2}\times 16^2=64\pi\]

Answer 3

If you have the diameter, halve it to get the radius...

Answer 4

And it doesn't give the diameter, it gives the radius, and a right-angle... i'm sure you know what a right angle is by now.