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# 1+2+3+..+n

sub sum_natural($n) {
    my ($total,$k)=0,1;
    while $k<=$n {
	$total=$total+$k;
	$k=$k+1;
    }
    return $total;
}

#1^3+2^3+..+n^3

sub sum_cube($n) {
    my ($total,$k)=0,1;
    while $k<=$n {
	$total=$total+$k**3;
	$k=$k+1;
    }
    return $total;
}

#
sub sum_pi($n) {
    my ($total,$k)=0,1;
    while $k<=$n {
	$total=$total+8/($k*($k+2));
	$k=$k+4;
    }
    return $total;
}