Unformatted text preview: ve. Hence, |a| = −a, |b| = −b,
and |ab| = ab. The equation |ab| = |a||b| becomes ab = (−a)(−b), which is true. Suppose a is
positive and b is negative. Then ab is negative, and we have |ab| = −ab, |a| = a and |b| = −b.
The equation |ab| = |a||b| reduces to −ab = a(−b) which is true. A symmetric argument shows the
equation |ab| = |a||b| holds when a is negative and b is positive. Finally, if either a or b (or both)
are zero, then both sides of |ab| = |a||b| are zero, and so the equation holds in this case, too. All
of this rhetoric has shown that the equation |ab| = |a||b| holds true in all cases. The proof of the
Quotient Rule is very similar, with the exception that b = 0. The Power Rule can be shown by
repeated application of the Product Rule. The last three properties can be proved using Deﬁnition
2.4 and by looking at the cases when x ≥ 0, in which case |x| = x, or when x < 0, in which case
|x| = −x. For example, if c > 0, and |x| = c, then if x ≥ 0, we have x = |x| = c. If, on the other
hand, x < 0, then −x = |x| = c, so x = −c. The remaining properties are proved similarly and are
left for the exercises.
To graph functi...
View Full Document