Exercise 7.3/39(b) - Recall that an angle measured in radians 
    is defined as follows: put the vertex at the center 
    of a circle of radius R, and let the length of the arc 
    that the angle cuts from the circumference of the circle be S, 
    then the angle in radians is equal to S/R.  
    Since the circumference of the whole circle is 2*pi*R, 
    the whole circle (i.e., 360 degrees) is equal to 
                  S     2*pi*R
                 --- = -------- = 2*pi   radians .
                  R       R 
    Since the area of the whole circle is pi*R^2, 
    the area of a sector of the circle cut by an angle theta 
    (a sector is the shape cut by the angle, like a slice of pizza) is 

                  theta
                 ------- * (the area of the whole circle) 
                   2*pi

                  theta              theta
               = ------- * pi*R^2 = ------- * R^2 .
                   2*pi                2

     Finally, how can you express theta in terms of x in the figure 
     in Exercise 7.2/39(b)?  (The answer will give you the fist term 
     in the expression for the definite integral.)