Download 1.4 Rational Expressions Definition 1.1 A quotient of two algebraic

1.4
Rational Expressions
Definition 1.1 A quotient of two algebraic expressions is called a fractional expression.
A rational expression is a fractional expression where both the numerator and denominator are polynomials.
√
For example,
x+1
x−2
is
a
fractional
expression
while
is a rational expression.
x2 + 4x
x2 − 5x + 6
The Domain of an Algebraic Expression
Definition 1.2 The domain of an algebraic expression is the set of values that the variable
is allowed to take.
Examples:
1
1. The expression
has domain {x | x 6= 0}, that is, the set of all numbers except 0.
x
This is because you cannot divide by 0.
√
2. The expression x has domain {x | x ≥ 0}, that is, the set of all non-negative numbers.
This is because you cannot take the square root of a negative number.
1
3. The expression √ has domain {x | x > 0}, that is, the set of all numbers except
x
0. This is because you cannot take the square root of a negative number nor can you
divide by 0.
4. Find the domain of 2x2 + 3x − 1.
solution: Here x is allowed to take any value, so the domain is all real numbers, R.
x
5. Find the domain of
(x − 2)(x − 3)
solution: Here, x is allowed to take any value except 2 and 3 since both of these would
make the bottom zero. Hence the domain is “all real numbers except 2 and 3” or,
written in set notation, {x | x 6=, x 6= 3}.
√
x
6. Find the domain of
(x − 5)
solution: The square root makes it so we can’t let x be any negative number, and the
x − 5 in the bottom makes it so that x cannot be 5, so in set notation our domain is
{x | x 6= 5 and x ≥ 0}.
Simplifying Rational Expressions
Just like when dealing with regular fractions, you can cancel a common term from the
numerator and denominator as long as it is not zero:
AC
if C 6= 0
BC
Example:
x+1
x2 − 1
(x−1)(x
+ 1)
=
=
2
x +x−2 (x− 1)(x + 2)
x+2
if x 6= 1
Multiplying and Dividing Rational Expressions
Multiplying and dividing rational expressions is done in an almost identical way to multiplying and dividing ordinary fractions.
A C
AC
Multiplication:
·
=
B D
BD
Example:
(x − 1)(x + 3) x + 4
x+3
x2 + 2x − 3 x + 4
·
=
·
=
2
2
x + 8x + 16 x − 1
(x + 4)
x−1
x+4
Division:
A C
A D
÷
= ·
B D
B C
Example:
x+3
x − 4 x2 − 3x − 4
x − 4 x2 + 5x + 6
(x + 3)
x
−
4
(x+2)
=
÷ 2
= 2
· 2
=
·
2
(x− 4)(x + 1)
x − 4 x + 5x + 6
x − 4 x − 3x − 4
(x − 2)
(x+ 2) (x − 2)(x + 1)
Adding and Subtracting Rational Expressions
To add or subtract rational expressions, they need to have the same denominator. If they
do not, you must first put them under the same denominator before adding or subtracting
them:
A C
A D C B
AD + BC
+
= · + · =
B D
B D D B
BD
Examples:
1.
3
x
3(x + 2)
x(x − 1)
+
=
+
x−1 x+2
(x − 1)(x + 2) (x + 2)(x − 1)
3x + 6 + x2 − x
=
(x − 1)(x + 2)
x2 + 2x + 6
=
(x + 2)(x − 1)
2.
1
1
x2 (x2 − 1)
2
−x = 2
−
x2 − 1
x −1
x2 − 1
4
2
1−x +x
=
x2 − 1
−x4 + x2 + 1
=
x2 − 1
3. Simplify the expression
x
y
+1
1−
y
x
.
solution:
x
y
+1
y
x
1−
4. Simplify the expression
=
x+y
y
x−y
x
=
x+y
x
x(x + y)
·
=
y
x−y
y(x − y)
1
a+h
− a1
.
h
solution:
1
a+h
−
h
1
a
=
a−(a+h)
a(a+h)
h
=
a−a−h 1
−1
· =
a(a + h) h
a(a + h)
(1 + x2 )1/2 − x2 (1 + x2 )−1/2
1 + x2
solution: We should first factor out (1 + x2 )−1/2 from the top, as it is the lowest power
of that factor present.
5. Simplify the expression
(1 + x2 )−1/2 ((1 + x2 ) − x2 )
(1 + x2 )1/2 − x2 (1 + x2 )−1/2
=
1 + x2
1 + x2
2
1 + x − x2
=
(1 + x2 )1/2 (1 + x2 )
1
=
(1 + x2 )1+1/2
1
=
(1 + x2 )3/2
Rationalizing the Denominator
√
In order to rationalize a function with the denominator in the form A + B C we multiply
√
the numerator and denominator by its conjugate A − B C.
√
√
√
√
1
1− 2
1− 2
1− 2
1
1− 2 √
√ =
√ ·
√ =
√
√ =
Example:
=
= 2−1
(1 − 2)
−1
1+ 2
1+ 2 1− 2
(1 − 2)(1 + 2)
As a final note, here are some errors that commonly befall students. Be careful!
1. (a + b)2 6= a2 + b2
√
√
√
2. a + b 6= a + b
√
3. a2 + b2 6= a + b
4.
1 1
1
+ 6=
a b
a+b
5.
a+b
a
6= b
6. a−1 + b−1 6= (a + b)−1
1.5
Equations and Inequalities
Definition 1.3 An equation is a statement that two mathematical expressions are equal.
Solving an equation is to find all values for all the unknowns that make the equation true.
An inequality is a statement that two mathematical expressions are <, >, ≤, or ≥ each
other. Solving an inequality is to find all values for the unknowns that make the inequality
true.
Examples:
1. x + 1 = 0. The unknown is x and has one solution, x = −1.
2. Solve the equation 4x + 7 = 19. solution:
4x + 7 = 19
4x = 19 − 7
12
4x
=
4
4
x = 3
Definition 1.4 A linear equation in one variable is an equation equivalent to one of the
form ax + b = 0 where a and b are real numbers and x is the variable.
Example: 4x − 5 = 0 is linear while x2 + 2x = 8 is not.
If a = 0 solving a linear equation is either impossible (if b 6= 0) or trivial (0 = 0). If a 6= 0,
then the solution of ax + b = 0 is x = − ab .
1. Solve the equation 7x − 4 = 3x + 8.
solution: We isolate the variable by moving all the terms involving x to one side and
everything else to the other side.
7x − 4 = 3x + 8
7x − 3x = 8 + 4
4x = 12
12
x =
4
x = 3
2. Solve the inequality 4x + 17 ≤ 19
solution: We can solve inequalities in much the same way that we solve equalities:
4x + 17 ≤ 19
4x ≤ 19 − 17
4x ≤ 2
1
x ≤
2
So the set of x that satisfy the inequality is all x ≤ 12 . In set notation, the solution is
{x | x ≤ 12 } and in interval notation the solution is (−∞, 12 ].
Note: we solve inequalities in much the same way as equations with one key difference multiplying each side of an inequality by the same negative number reverses the direction
of the inequality.
Example: Solve the inequality 3x < 9x + 4.
solution:
3x < 9x + 4
3x − 9x < 4
−6x < 4 1
1
(−6x) > 4 −
−
6
6
2
x > −
3
Again, the set of all solutions can be written as {x | x > − 32 } or (− 32 , ∞).
Rules of Inequalities
1. A ≤ B ⇔ A + C ≤ B + C.
2. A ≤ B ⇔ A − C ≤ B − C.
3. If C is positive and A ≤ B, then AC ≤ BC and vice-versa.
4. If C is negative and A ≤ B, then AC ≥ BC and vice-versa.
5. If A and B are both positive and A ≤ B, then
1
A
≥
1
B
and vice-versa.
6. If A ≤ B and C ≤ D, then A + C ≤ B + D.
Definition 1.5 A quadratic equation is an equation of the form ax2 + bx + c = 0 where
a, b and c are real numbers with a 6= 0. Solutions of this type of equation are called the
zeroes or roots of the polynomial ax2 + bx + c.
In cases where the quadratic can be factored, finding the solutions is relatively easy. For
example, if we wanted to solve x2 + 5x + 6 = 0. We would notice that x2 + 5x + 6 =
(x + 3)(x + 2) and hence our equation is
(x + 3)(x + 2) = 0
Since we have two terms multiplied together to give 0, we know that one of the terms must
be zero. Hence, either x + 3 = 0 or x + 2 = 0 or, in other words, x = −2 or −3. Thus the
roots of x2 + 5x + 6 are −2 and −3.
If the quadratic cannot easily be factored, then to find the solutions you should use the
quadratic equation: if ax2 + bx + c = 0, then the solutions are
√
−b ± b2 − 4ac
x=
.
2a
In this case, we have that ax2 + bx + c must factor as follows:
√
√
2 − 4ac
−b
+
b
−b
−
b2 − 4ac
2
ax + bx + c = x −
x−
2a
2a
Example: Solve 3x2 − 5x − 1 = 0.
solution: a = 3, b = −5 and c = −1, so we have
p
√
√
−(−5) ± (−5)2 − 4(3)(−1)
5 ± 25 + 12
5 ± 37
x=
=
=
2(3)
6
6
note that in this case there are two solutions, one from the plus and one from the minus.