Need to simplify: n! /(n-1)!

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Which expression is equivalent to (3^2)^-2 a.-81 b.-12 c.1/81 d.1/12

Molly shared a spool of ribbon with 12 people. each person received 3 feet of ribbon. which equation can she use to find r, the number of feet of ribbon that her spool originally had?

The area of a circle is equal to 1 dm². find the radius of the circle.