Question 1072934
No, you need to find when the value of the function was less than 6600.
{{{m(t)<6600}}}
{{{3200cos(0.72t-1.15)+6400<6600}}}
{{{3200cos(0.72t-1.15)<200}}}
{{{cos(0.72t-1.15)<0.0625}}}
So find the times when the value of the cosine function is equal to 0.0625,
{{{0.72t-1.15=1.5083}}}
{{{0.72t=2.658}}}
{{{t=3.692}}}
and
{{{0.72t-1.15=4.775}}}
{{{0.72t=5.925}}}
{{{t=8.229}}}
.
.
.
*[illustration fr31.JPG].
.
.
.
So between t=3.7 years and t=8.2 years, the number of pieces of mail fell below 6600.