In Exercises 21 and 22, write the statement as a power function equa- tion, Let k be the constant of variation. 22. The force of gravity F acting on an object is inversely propor- tional to the square of the distance d from the object to the center of Earth. In Exercises 23 and 24, write a sentence that expresses the relationship in the formula, using the language of variation or proportion. 23. F = kx, where F is the force it takes to stretch a spring x units from its unstressed length and k is the spring's force constant. 24. A = T . 12, where A and r are the area and radius of a circle and 7 is the usual mathematical constant. In Exercises 25-28, state the values of the constants k and a for the function f(x) = k . x". Describe the portion of the curve that lies in Quadrant I or IV. Determine whether f is even, odd, or undefined for x