16. Use a suitable change of variables to determine the indefinite integral. dx 1 + 17. The area of a semicircle of radius 1 can be expressed as V1 - x2dx Use the substitution x = cost to express the area of a semicircle as the integral of a trigonometric function. You do not need to compute the integral. 18. Find each indefinite integral by using appropriate substitution. [cosx - xsinx dx xCOSx 19. Find each indefinite integral by using appropriate substitution. Jetand sec2 xdx 20. Verify by differentiation that Inx dx = x(In x - 1) + C, then use appropriate changes of variables to compute the integral. x 2 In ( x 2 ) dx