a) A particle travels in a straight line with the position function s(t) = t² + 4t+4.9, t 0. At one instant on the interval [0,4] the instantaneous velocity is equal to the average velocity on that interval. Find that value of t (and enter it in the box below)

b) The equation y = (4²-x²)^1/2 describes a semicircle centered at the origin of radius 4. Show that the line tangent to any point on the semicircle is perpendicular to the radius drawn to that point.