**Exercise 1.39:** A continued fraction representation of the tangent function was published in 1770 by the German
mathematician J.H. Lambert:

$$ \tan x = \cfrac{x}{1-\cfrac{x^2}{3-\cfrac{x^2}{5-{\ddots,}}}} $$

where `x`

is in radians. Define a procedure `(tan-cf x k)`

that computes an approximation to the tangent
function based on Lambert’s formula. `K`

specifies the number of terms to compute, as in exercise 1.37.

```
(define (tan-cf x k)
(define (n i)
(if (= i 1)
x
(* x x)))
(define (d i)
(- (* 2 i) 1))
(define (cf i)
(if (= i (+ k 1))
0
(/ (n i)
(- (d i) (cf (+ i 1))))))
(if (not (> k 0))
0
(cf 1)))
```

```
> (tan-cf (sqrt 2) 12)
6.334119167042199
```

$$ \tan \sqrt 2 = 6.3341191670421915540568332642277 $$