6. 局所変数 (define (quadric-equation a b c) (cond ((zero? a) 'error) (else (let ((d (- (* b b)(* 4 a c)))) (cond ((negative? d)'()) (else (let ((e (/ b a -2))) (cond ((zero? d)(list e)) (else (let ((f (/ (sqrt d) a 2))) (list (+ e f)(- e …