Vlad is playing on a swing set. His horizontal distance D(t)D(t)D, left parenthesis, t, right parenthesis (in \text{m}mstart text, m, end text) from the center (where being behind the center means a negative distance) as a function of time ttt (in seconds) can be modeled by a sinusoidal expression of the form a\cdot\cos(b\cdot t)+da⋅cos(b⋅t)+da, dot, cosine, left parenthesis, b, dot, t, right parenthesis, plus, d. At t=0t=0t, equals, 0, when he pushes off, he is 1\text{ m}1 m1, start text, space, m, end text behind the center, which is as far back as he goes. The swing reaches the center \dfrac{\pi}{6} 6 π start fraction, pi, divided by, 6, end fraction seconds later.