Eva (and W.Z.) is right. We need to distinguish between dependent and independent intervals. In the formula for resistance and are independent intervals (they correspond to resistance of physically different resistors), and in procedure **par1** two occurrences of are dependent intervals. When we calculate the ratio we cannot suppose that those two intervals are independent, we cannot choose their values independently. If we randomly choose the value inside the interval , we must assign this value to both occurrences of in the formula. For independent intervals the situation is different, we may choose one value inside , some other value inside , and their ratio will have meaning, namely this will be some value inside the interval which is the ratio of two intervals, . Alyssa’s package treats all occurrences of the same interval in a complex formula as independent intervals and the answers are wrong. The calculation of by Alyssa’s package gives wrong result.

# SICP Exercise 2.15

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