(define pi 3.14159265358979)
(define (in x) (- (log (sin (+ (/ x 2)(/ pi 4)))) (log (sin (- (/ x 2)(/ pi 4))))))
(define (f x) (/ 1 (cos x)))
(define (dv f)(let ((dx .001))(lambda (x)(/ (- (f (+ x dx)) (f x)) dx))))
(define (In x)(log (tan (+ (/ x 2) (/ pi 4)))))
(f .714) ; => 1.3231915447760345
((dv In) .714) ; => 1.3237653748630063
(f 1) ; => 1.8508157176809255
((dv In) 1) ; => 1.8522587622462616