Algebra Errors
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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 thata 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:
which is a bit complicated and confuses many beginners. Perhaps it's better to start with the distance concept.
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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,
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 thatsin(x+y)=sin(x)+sin(y) is correct.
The "everything is additive" error is actually the most common occurrence of a more general class of errors:
Everything is commutative. In higher mathematics, we say that two operations commute if we can perform them in either order and get the same result. We've already looked at some examples with addition; here are some examples with other operations. Contrary to some students' beliefs,
etc. Another common error is to assume that multiplication commutes with differentiation or integration. But actually, in general (uv)′ does not equal (u′)(v′) and ∫ (uv) does not equal (∫ u)(∫ v).
However, to be completely honest about this, I must admit that there is one very special case where such a multiplication formula for integrals is correct. It is applicable only when the region of integration is a rectangle with sides parallel to the coordinate axes, and
u(x) is a function that depends only on x (not on y), and
v(y) is a function that depends only on y (not on x).
Under those conditions,
(I hope that I am doing more good than harm by mentioning this formula, but I'm not sure that that is so. I am afraid that a few students will write down an abbreviated form of this formula without the accompanying restrictive conditions, and will end up believing that I told them to equate ∫ (uv) and (∫ u)(∫ v) in general. Please don't do that.)
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Undistributed cancellations. Here is an error that I have seen fairly often, but I don't have a very clear idea why students make it.
(3x+7)(2x–9) + (x2+1) (3x+7) (2x–9) + (x2+1) (2x–9) + (x2+1)
f(x) = ________________________________________ ________________________________________ = ________________________________________
(3x+7)(x3+6) (3x+7) (x3+6) (x3+6)
In a sense, this is the reverse of the "loss of invisible parentheses" mentioned earlier; you might call this error "insertion of invisible parentheses." To see why, compare the preceding computation (which is wrong) with the following computation (which is correct).
(3x+7) [ (2x–9) + (x2+1)] (3x+7) [ (2x–9) + (x2+1) ] (2x–9) + (x2+1)
g(x) = ________________________________________ = ________________________________________ = ________________________________________
(3x+7) (x3+6) (3x+7) (x3+6) (x3+6)
Apparently some students think that f(x) and g(x) are the same thing -- or perhaps they simply don't bother to look carefully enough at the top line of f(x), to discover that not everything in the top line of f(x) has a factor of (3x+7). If you still don't see what's going on, here is a correct computation involving that first function f :
x2+1
2x–9 + ________________________________________
(3x+7)(2x–9) + (x2+1) 3x+7
f(x) = ________________________________________ = ________________________________________
(3x+7)(x3+6) x3+6
Why would students make errors like these? Perhaps it is partly because they don't understand some of the basic concepts of fractions. Here are some things worth noting:
• Multiplication is commutative -- that is, xy = yx. Consequently, most rules about multiplication are symmetric. For instance, multiplication distributes over addition both on the left and on the right:
(x1+x2)y=(x1y)+(x2y) and x(y1+y2)=(xy1)+(xy2) .
• Division is not commutative -- in general, x/y is not equal to y/x. Consequently, rules about division are not symmetric (though perhaps some students expected them to be symmetric). For instance,
(x1+x2)/y = (x1/y)+(x2/y) but in general x/(y1+y2) ≠ (x/y1)+(x/y2) .
• Fractions represent division and grouping (i.e., parentheses). For instance, the fraction
a+b
c+d
• is the same thing as (a+b)/(c+d). If you omit either pair of parentheses from that last expression, you get something entirely different. (Thanks to Mark Meckes for pointing out this possible explanation of the origin of such errors.) Perhaps some of the students' errors stem from such an omission of parentheses? or a lack of understanding of how important those parentheses are? That would seem to be indicated by the prevalance of another type of error described elsewhere on this page, "loss of invisible parentheses".
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Dimensional errors. Most of this web page is devoted to things that you should not do, but dimensional analysis is something that you should do. Dimensional analysis doesn't tell you the right answer, but it does enable you to instantly recognize the wrongness of some kinds of wrong answers. Just keep careful track of your dimensions, and then see whether your answer looks right. Here are some examples:
• If you're asked to find a volume, and your answer is some number of square inches, then you know you've made an error somewhere in your calculations. (If you find this kind of error in your answer,don't just change "square inches" to "cubic inches" in your answer and leave the numerical part unchanged. The step in your calculation where you got the wrong units may also be a step where you made a numerical error. Try to find that step,)
• If you're asked to find an area or a volume, and your answer is a negative number, then you know you've made an error somewhere. (Again, don't just change the sign in your answer -- there may be more to your error than that.)
• If the question is a word-problem, think about whether your answer makes sense. For instance, if you're given the dimensions of a coin and you're asked to find its surface area, and you come up with an answer of 3000 square miles, you should realize that you've probably made an error (even though your answer has the right units), and you should look for that error. (This is not really an example of dimensional analysis, but I didn't know where else to put it. Thanks to Sandeep Kanabar for this example.)
• Even if yo