T = 7 D = 9 d = t * (T - t) d = tT - t² for which range of t is d > D? 0 >= t >= T 10 = t7 - t² 10 = - t² + t7 -10 = t² - 7t 3.5² - 10 = t² - 7t + 3.5² 49/4 - 10 = t² - 7t + 49/4 9/4 = (t - 7/2)² t - 7/2 = 3/2 or -3/2 t = 10/2 or 4/2 5, 2 (5 + 1) - 2 = 4