D(t)D, left parenthesis, t, right parenthesis models the distance (in thousands of \text{km}kmstart text, k, m, end text) from the earth to the Moon ttt days after the moon's perigee (when it's closest to the earth). Here, ttt is entered in radians. D(t) = -21\cos\left(\dfrac{2\pi}{29.5}t\right) + 384D(t)=−21cos( 29.5 2π t)+384D, left parenthesis, t, right parenthesis, equals, minus, 21, cosine, left parenthesis, start fraction, 2, pi, divided by, 29, point, 5, end fraction, t, right parenthesis, plus, 384 How many days after its perigee does the moon first reach 380380380 thousands of \text{ km} kmstart text, space, k, m, end text from the Earth?

[tex]D(t) = -21\cos\left(\dfrac{2\pi}{29.5}t\right) + 384\\\\380=-21\cos\left(\dfrac{2\pi}{29.5}t\right)+384\\\\\dfrac{4}{21}=\cos\left(\dfrac{2\pi}{29.5}t\right)\qquad\text{subtract 384, divide by -21}\\\\\dfrac{2\pi}{29.5}t=\arccos{\dfrac{4}{21}}\qquad\text{use the inverse cosine function}\\\\t=\dfrac{29.5}{2\pi}\arccos{\dfrac{4}{21}}\approx 6.4752\quad\text{days}[/tex]

About 6.48 days after perigee, the moon reaches a distance of 380,000 km from Earth.