The unit circle is used to make the trig identities..........the functions............sine waves.............it is easy to see why its area is exactly pi............b/c its radius is one..........one squared is one.....

Circle

Solve for 

area

A

=

π

r

2

r Radius

Area of a circle (video) | Khan Academy

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https://www.khanacademy.org/...area.../area...circle/.../area-of-a-circ...

Area of a circle. ... The area of a circle is pi times the radius squared (A = π r²). Learn how to use this formula to ...