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<!DOCTYPE html>
<html>
<body>
<?php
echo(atanh(M_PI_4) . "<br>");
echo(atanh(0.50) . "<br>");
echo(atanh(-0.50) . "<br>");
echo(atanh(1) . "<br>");
echo(atanh(-1));
?>
</body>
</html>
1.0593061708232
0.54930614433405
-0.54930614433405
INF
-INF