Posted by **anna** on Friday, August 5, 2011 at 9:36pm.

Hi i posted this before but I wanted to ask Bobpursley a few questions on his reply

1)For the first function you used cosine when they ask for sin, why is that? Also why did you put .5 in the beginning and end of the function?

2)For the second question why did you put 10 in front of the cos instead of 20?

The sun always illuminates half of the moon’s surface, except during a lunar eclipse. The illuminated portion of the moon visible from Earth varies as it revolves around Earth resulting in the phases of the moon. The period from a full moon to a new moon and back to a full moon is called a synodic month and is 29 days, 12 hours, and 44.05 minutes long. Write a sine function that models the fraction of the moon’s surface which is seen to be illuminated during a synodic month as a function of the number of days, d, after a full moon. [Note: full moon equals illuminated.]

I am really confused on this one I know I have to make a sine function.

so would it be like y= 29sin(pi/6*t)+44.05

sorry that was probably a really bad guess.

2)A bicycle tire has a diameter of 20 inches and is revolving at a rate of 10 rpm. At t = 0, a certain point is at height 0. What is the height of the point above the ground after 20 seconds?

- Precalc(attn Bobpursley) -
**Damon**, Saturday, August 6, 2011 at 6:54am
Guessing because I have not seen original:

cos instead of sin because it begins with full moon, cos 0 = 1 but sin 0 = 0

The half of the moon opposite the sun remains dark. Only 1/2 the moon can be illuminated.

The diameter of the tire is 20, but the radius is 10 which is the dimension you want to use.

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