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```
my $ninety_degrees = atan(1)*2;
```

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sub atan {
    return 0 if $_[0]==0;
...
         atan2(sqrt(1+$_[0]*$_[0]),sqrt(1+1/($_[0]*$_[0]))) :
        -atan2(sqrt(1+$_[0]*$_[0]),sqrt(1+1/($_[0]*$_[0])));
}