add SICP exercise solutions

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diff --git a/SICP/ex2_7.scm b/SICP/ex2_7.scm
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+;; 2.1.4 Extended Exercise:Interval Arithmetic
+
+(define (add-interval x y)
+  (make-interval (+ (lower-bound x)
+                    (lower-bound y))
+                 (+ (upper-bound x)
+                    (upper-bound y))))
+
+(define (mul-interval x y)
+  (let ((p1 (* (lower-bound x)
+               (lower-bound y)))
+        (p2 (* (lower-bound x)
+               (upper-bound y)))
+        (p3 (* (upper-bound x)
+               (lower-bound y)))
+        (p4 (* (upper-bound x)
+               (upper-bound y))))
+    (make-interval (min p1 p2 p3 p4)
+                   (max p1 p2 p3 p4))))
+
+;; Todo:ensure this is right
+(define (div-interval x y)
+  (if (>= 0 (* (lower-bound y) (upper-bound y)))
+    (error "error!" y)
+    (mul-interval x
+                  (make-interval
+                    (/ 1.0 (upper-bound y))
+                    (/ 1.0 (lower-bound y))))))
+
+(define (make-interval a b) (cons a b))
+
+;; Exercise:2.7
+
+;; My version
+;; (define (lower-bound interval) (car interval))
+;; (define (upper-bound interval) (cdr interval))
+
+(define (lower-bound interval) (min (car interval)
+                                    (cdr interval)))
+(define (upper-bound interval) (max (car interval)
+                                    (cdr interval)))
+
+;; ???
+;; Exercise:2.8 
+(define (sub-interval x y)
+  (make-interval (- (lower-bound x) (upper-bound y))
+                 (- (upper-bound x) (lower-bound y))))
+
+(define (display-interval i)
+  (newline)
+  (display "[")
+  (display (lower-bound i))
+  (display ",")
+  (display (upper-bound i))
+  (display "]"))
+
+(define i (make-interval 1 5))
+(define j (make-interval 1 4))
+
+(display-interval i)
+(display-interval (sub-interval i j))
+(display-interval (sub-interval j i))
+
+;; Exercise:2.9
+
+(define (interval-width i)
+  (/ (- (upper-bound i) (lower-bound i))
+     2))
+
+(interval-width (make-interval 2 8))
+
+;; Exercise:2.10
+
+(define (div-interval x y)
+  (if (>= 0 (* (lower-bound y) (upper-bound y)))
+    (error "error!" y)
+    (mul-interval x
+                  (make-interval
+                    (/ 1.0 (upper-bound y))
+                    (/ 1.0 (lower-bound y))))))
+
+(define (make-interval a b) (cons a b))
+
+(define span-0 (make-interval -1 2))
+
+(display-interval (div-interval i j))
+(display-interval (div-interval i span-0))
+
+;; Exercise:2.11
+