I guess it depends of what the test is about.... I just can't picture how a book can help to solve a non linear control theory problem (Liapunov's criteria or something similar)..... no matter if you find the formula, if you don't know what the formula means or how it apllies to your problem, you are DOA..... Apply the same to most of the disciplines that require to solve a problem (solve a 2 degree partial differential equation, or get the roots that make a low band filter stable.....)
Usually, we were even given the formulas we needed on the board. No books or bags allowed in the clasroom. No restroom breaks, you were instructed to visit the porcelanite folk before the test.... Yes, in some cases you knew what to do and and how to solve it, but you were unsure about a term in a formula. In my "Control theory II" class (root locus, bode, nyquist....) I stumbled once in a problem I wasn't sure about a term in a formula (the teacher was really tough, no formulas were given, and only basic calculator were allowed in order to ensure the students were proefficient using their skills) Some guys needed help in the same problem, but I told them I couldn't help as I wasn't sure if my formula was accurate..... 5 minutes later a post- it note is placed in my back with the formula...... I confirm my doubt about the formula, solve the problem and then I write in another paper the basic that they needed to reach the 70/100 minimum to pass the test.
Something similar happened in "Analog Electronic IV" when working on a PLL circuit, but i think it happened no more that 3 times. I never considered it as cheating, as Albert Einstein himself said that "Imagination and Reasoning is more important than knowledge"....

In fact, fast response was key to make the runs sucesfully...
Alex