Question 1140761
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One part of the response from the other tutor needs to be corrected.<br>
To find the phase shift, you need to factor out the coefficient of x.<br>
{{{y=20cos(9x-81)-2}}}  -->  {{{y=20 cos(9(x-9))-2}}}<br>
The phase shift is 9 degrees, not 81 degrees.<br>
His explanation of how to determine the period is 2pi/9 (or 360/9 degrees) is a good one.  Here is another way to see that is the period.<br>
The cosine function takes on all its values in a range of 2pi radians.  So this function takes on all its values when 9x ranges from 0 to 2pi:<br>
{{{0 <= 9x <2pi}}}
{{{0 <= x < 2pi/9}}}  <--  the period is 2pi/9.