Algebra Errors
Sign errors are surely the most common errors of all. I generally deduct only one point for these errors, not because they are unimportant, but because deducting more would involve swimming against a tide that is just too strong for me. The great number of sign errors suggests that students are careless and unconcerned -- that students think sign errors do not matter. But sign errors certainly do matter, a great deal. Your trains will not run, your rockets will not fly, your bridges will fall down, if they are constructed with calculations that have sign errors.
Sign errors are just the symptom; there can be several different underlying causes. One cause is the "loss of invisible parentheses," discussed in a later section of this web page. Another cause is the belief that a minus sign means a negative number. I think that most students who harbor this belief do so only on an unconscious level; they would give it up if it were brought to their attention. [My thanks to Jon Jacobsen for identifying this error.]
Is –x a negative number? That depends on what x is.
Yes, if x is a positive number.
No, if x itself is a negative number. For instance, when x = –6, then –x = 6 (or, for emphasis, –x = +6).
That's something like a "double negative". We sometimes need double negatives in math, but they are unfamiliar to students because we generally try to avoid them in English; they are conceptually complicated. For instance, instead of saying "I do not have a lack of funds" (two negatives), it is simpler to say "I have sufficient funds" (one positive).
Another reason that some students get confused on this point is that we read "–x" aloud as "minus x" or as "negative x". The latter reading suggests to some students that the answer should be a negative number, but that's not right. [Suggested by Chris Phillips.]
Misunderstanding this point also causes some students to have difficulty understanding the definition of the absolute value function. Geometrically, we think of |x| as the distance between x and 0. Thus |–3| = 3 and |27.3| = 27.3, etc. A distance is always a positive quantity (or more precisely, a nonnegative quantity, since it could be zero). Informally and imprecisely, we might say that the absolute value function is the "make it positive" function.
Those definitions of absolute value are all geometric or verbal or algorithmic. It is useful to also have a formula that defines |x|, but to do that we must make use of the double negative, discussed a few sentences ago. Thus we obtain this formula:
[image: absolute value of x is
x if x is greater than or equal
to 0, or -x if x is less than 0]
which is a bit complicated and confuses many beginners. Perhaps it's better to start with the distance concept.
Many college students don't know how to add fractions. They don't know how to add (x/y)+(u/v), and some of them don't even know how to add (2/3)+(7/9). It is hard to classify the different kinds of mistakes they make, but in many cases their mistakes are related to this one:
Everything is additive. In advanced mathematics, a function or operation f is called additive if it satisfies f(x+y)=f(x)+f(y) for all numbers x and y. This is true for certain familiar operations -- for instance,
the limit of a sum is the sum of the limits,
the derivative of a sum is the sum of the derivatives,
the integral of a sum is the sum of the integrals.
But it is not true for certain other kinds of operations. Nevertheless, students often apply this addition rule indiscriminately. For instance, contrary to the belief of many students,
[image:
sin(x+y) is NOT equal to sin x+sin y,
(x+y)^2 is NOT equal to x^2+y^2,
sqrt(x+y) is NOT equal to sqrt x+sqrt y,
1/(x+y) is NOT equal to (1/x)+(1/y).]
We do get equality holding for a few unusual and coincidental choices of x and y, but we have inequality for most choices of x and y. (For instance, all four of those lines are inequalities when x = y = π/2. The student who is not sure about all this should work out that example in detail; he or she will see that that example is typical.)
One explanation for the error with sines is that some students, seeing the parentheses, feel that the sine operator is a multiplication operator -- i.e., just as 6(x+y)=6x+6y is correct, they think that sin(x+y)=sin(x)+sin(y) is correct.