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## 1.9 Chapter summary (EMAR)

Presentation: 2DR6

• $$\mathbb{N}$$: natural numbers are $$\left\{1; 2; 3; \ldots\right\}$$

• $$\mathbb{N}_0$$: whole numbers are $$\left\{0; 1; 2; 3; \ldots\right\}$$

• $$\mathbb{Z}$$: integers are $$\left\{\ldots; -3; -2; -1; 0; 1; 2; 3; \ldots\right\}$$

• A rational number is any number that can be written as $$\frac{a}{b}$$ where $$a$$ and $$b$$ are integers and $$b\ne 0$$.

• The following are rational numbers:

• Fractions with both numerator and denominator as integers

• Integers

• Decimal numbers that terminate

• Decimal numbers that recur (repeat)

• Irrational numbers are numbers that cannot be written as a fraction with the numerator and denominator as integers.

• If the $$n^{\text{th}}$$ root of a number cannot be simplified to a rational number, it is called a surd.

• If $$a$$ and $$b$$ are positive whole numbers, and $$a<b$$, then $$\sqrt[n]{a}<\sqrt[n]{b}$$.

• A binomial is an expression with two terms.

• The product of two identical binomials is known as the square of the binomial.

• We get the difference of two squares when we multiply $$\left(ax+b\right)\left(ax-b\right)$$

• Factorising is the opposite process of expanding the brackets.

• The product of a binomial and a trinomial is:

$\left(A+B\right)\left(C+D+E\right)=A\left(C+D+E\right)+B\left(C+D+E\right)$
• Taking out a common factor is the basic factorisation method.

• We often need to use grouping to factorise polynomials.

• To factorise a quadratic we find the two binomials that were multiplied together to give the quadratic.

• The sum of two cubes can be factorised as: ${x}^{3}+{y}^{3}=\left(x+y\right)\left({x}^{2}-xy+{y}^{2}\right)$

• The difference of two cubes can be factorised as: ${x}^{3}-{y}^{3}=\left(x-y\right)\left({x}^{2}+xy+{y}^{2}\right)$

• We can simplify fractions by incorporating the methods we have learnt to factorise expressions.

• Only factors can be cancelled out in fractions, never terms.

• To add or subtract fractions, the denominators of all the fractions must be the same.