1 (define (par1 r1 r2)
2   (div-interval (mul-interval r1 r2)
3 		(add-interval r1 r2)))
4 (define (par2 r1 r2)
5   (let ((one (make-interval 1 1)))
6     (div-interval one
7 		  (add-interval (div-interval one r1)
8 				(div-interval r2)))))