4. Let r(t) = (=t3, t2, 2t) be a given vector function: a) For the portion of the corresponding space curve that goes from the origin to the point (72, 36, 12), find the exact length (4 points). b) At the point (9, 9, 6), find T(t), the unit tangent vector (4 points). c) At the point (9, 9, 6), compute the curvature, using any appropriate curvature formula discussed in class (4 points)