# SICP Exercise 2.15

Eva (and W.Z.) is right. We need to distinguish between dependent and
independent intervals. In the formula for resistance \(R_1\) and
\(R_2\) are independent intervals (they correspond to resistance
of physically different resistors), and in procedure `par1`

two
occurrences of \(R_1\) are dependent intervals. When we calculate
the ratio \(A/A\) we cannot suppose that those two intervals
\(A\) are independent, we cannot choose their values
independently. If we randomly choose the value inside the interval
\(A\), we must assign this value to both occurrences of \(A\) in the formula.
For independent intervals the situation is
different, we may choose one value inside \(A\), some other value
inside \(B\), and their ratio will have meaning, namely this will
be some value inside the interval which is the ratio of two intervals,
\(A/B\). 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 \(A/A\) by Alyssa's package gives
wrong result.