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#### Notes by Category University Engineering

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# Indices & Algebraic Methods

When working with indices, there are eight laws that must be followed:

These can be used to factorise and expand expressions.

### Surds

Surds are examples of irrational numbers, meaning they do not follow a repeating pattern but go on forever, uniquely. Pi is the most common example of an irrational number, but surds are slightly different - they are the square roots of non-square numbers.

âˆš4 = 2 4 is a square number, so gives a rational square root
âˆš2 = 1.4142... 2 is not a square number, so its square root is irrational

Like with indices, there are rules that apply to surds:

These can be used to rationalise denominators:

• For fractions in the form 1 / âˆša, multiply both numerator and denominator by âˆša

• For fractions in the form 1 / (a + âˆšb), multiply both numerator and denominator by (a - âˆšb)

• For fractions in the from 1 / (a - âˆšb), multiply both numerator and denominator by (a + âˆšb)

This is known as the conjugate pair (switching the sign of the denominator)

## Algebraic Fractions

To simplify algebraic fractions, factorise whatever can be factorised so that parts of the numerator and denominator can cancel:

### Multiplication

To multiply fractions, any common factors can be cancelled before multiplying the numerators and denominators.

### Division

To divide fractions, multiply the first fraction by the reciprocal of the second fraction (flip the second fraction).

To add or subtract one fraction from another, a common denominator must be found.

## Long Division of Polynomials

Polynomials are expressions that contain only rational numbers, positive indices and/or variables in the numerators.

• 3x + 5 and 3xÂ² + 5x + 7 are examples of polynomials

• 3/x, âˆšx and 5x-Â² are not polynomials

Polynomials can be divided by (x Â± p), where p is a constant, using long division:

### The Factor Theorem

The example above divides perfectly - it does not have a remainder. This means that (2x+1) must be a factor of the initial expression we divided it into. There is a quicker way to check this: the factor theorem.

The factor theorem states that if f(x) is a polynomial, then:

• if f(p) = 0, then (x-p) is a factor of f(x)

• if (x-p) is a factor of f(x), then f(p) = 0

### The Remainder Theorem

This can be used to find the remainder of a long division, without actually doing the division.

If (x-a) is not a factor of f(x), then the remainder is given as f(a)

## Partial Fractions

If a fraction has two or more distinct factors in its denominator, it can be separated into partial fractions.

### Three Linear Factors

This method cannot be used if two of the factors are the same (repeated)

### Improper Partial Fractions

An improper algebraic fraction is a fraction where the numerator has an equal or higher power to the denominator. These must first be converted into proper fractions before they can be expressed as partial fractions. There are two methods of doing this:

1. Use algebraic long division and add the remainder divided by the divisor to the quotient

2. Multiply by the divisor and add the remainder

You can find the remainder using the remainder theorem.

## Mathematical Proof

A mathematical proof is a logical argument to show that a conjecture (a mathematical statement) is always true. Typically, a proof begins with a theorem (a pre-established fact).

There are a number of requirements for a valid mathematical proof:

• All information and assumptions being used must be stated

• Every step must be shown explicitly

• Every step must lead on logically from the previous step

• All cases must be covered

• A proof must always end in a statement of proof.

To prove an identity, like (a+b)(a-b) â‰¡ aÂ² - bÂ², you start with the expression on one side, and manipulate it algebraically until it is exactly the same as the other side. Again, the requirements above apply.

### Proof by Deduction

Proof by deduction means starting from a fact or definition and using logical steps to prove the conjecture.

Conjecture:

• The product of two odd numbers is also odd

Proof:

• If a and b are integers, then 2a+1 and 2b+1 are definitely odd integers

• (2a+1)(2b+1) = 4ab + 2a + 2b +1 = 2(2ab+a+b) + 1

• This is in the same form as 2m+1, the standard form for an odd number

• Therefore, the product of two odd numbers is always odd

### Proof by Exhaustion

This involves breaking a proof into smaller proofs and dealing with these all individually. Since it requires every possible instance to be calculated, it is impossible for an infinite range (such as the example above) but can only be used on smaller scales between set limits.

Conjecture:

• The sum of perfect cubes between zero and 100 is a multiple of 10.

Proof:

• The only perfect cubes between zero and 100 are 1, 8, 27 and 64

• 1 + 8 + 27 + 64 = 100

• 100 = 10(10)

• Therefore, the sum of cubes between zero and 100 is a multiple of 10.

### Proof by Counter-Example

Perhaps the simplest form of mathematical proof (though often the most frustrating), proof by counter-example works by finding a single occurrence when the conjecture is false. If the conjecture is false once, it is always false.

Conjecture:

• All even multiples of five are also multiples of 4

Counter-Example:

• 5x2 = 10

• 10 is even

• 10 is not divisible by 4

• Therefore, not all even multiples of five are also multiples of four

Proof by contradiction works by first assuming the conjecture is untrue. Then, you show through logical steps that this is impossible, and so you conclude that the initial conjecture is in fact true.

The contradiction can either be with the initial assumption, or something else that is known to be true.

Conjecture:

• âˆš2 is irrational

Assumption:

• âˆš2 is not irrational

• Rational numbers can be expressed in the form a/b, where a and b have no common factors

• So, âˆš2 can be written as a/b: âˆš2 = a/b

• Squaring both sides gives 2 = aÂ²/bÂ²

• This can be rearranged to give aÂ² = 2bÂ²

• This means that aÂ² must be even, and so a is also even, and can be expressed as 2n (where n is an integer)

• Therefore, (2n)Â² = 2bÂ²

• This equals 4nÂ² = 2bÂ², which cancels to 2nÂ² = bÂ²

• This shows that bÂ² is also even, and so is b

• If a and b are both even, they have a common factor, 2

• This contradicts the statement that a and b have no common factors, so âˆš2 must be irrational

See All

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