연습문제 1.39

function cont_frac(n, d, k){
    function frac(x){
        return x > k ?
            : 0
            ? n(x) / ( d(x) + frac(x+1) ); 
    }
    return frac(1);
}

function tan_cf(x, k){
    return cont_frac(i => i === 1 ? x : x * x, i => 2i - 1, k )
}