Question 1204408
<pre>
{{{drawing(400,400,-5,53,-29,29,line(-10,15,65,15),
graph(400,400,-5,53,-29,29,( 20sin((pi/13)x)+2 )*(sqrt(x)/sqrt(x))*(sqrt(52-x)/sqrt(52-x))))}}}

{{{20sin(expr(pi/13)x) + 2 = 15}}}

{{{20sin(expr(pi/13)x) = 13}}}

{{{sin(expr(pi/13)x) = 13/20}}}

{{{0.2416609734x = 0.7075844367+2pi*n}}} or {{{0.2416609734x = 2.434008217+2pi*n}}}
{{{x = 0.7075844367/0.2416609734+26n}}} or {{{x = 2.43400817/0.2416609734+26n}}}

{{{x = 2.928004579+26n}}} or {{{x = 10.07199542+26n}}}

It starts being above 15<sup>o</sup> on week 2.9 from the first equation when n=0 
and stops being above 15<sup>o</sup> on week 10.1 from the second equation when n=0.

It starts being above 15<sup>o</sup> on week 28.9 from the first equation when n=1 
and stops being above 15<sup>o</sup> on week 36.1 from the second equation when n=1.

Edwin</pre>