Get your own PHP server Result Size: 625 x 565
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<!DOCTYPE html>
<html>
<body>
<?php
echo(acos(0.64) . "<br>");
echo(acos(-0.4) . "<br>");
echo(acos(0) . "<br>");
echo(acos(-1) . "<br>");
echo(acos(1) . "<br>");
echo(acos(2));
?>
</body>
</html>
0.87629806116834
1.9823131728624
1.5707963267949
3.1415926535898
0
NAN