diff --git a/docs/mhf4u7.md b/docs/mhf4u7.md index a8a1d65..f31a18e 100644 --- a/docs/mhf4u7.md +++ b/docs/mhf4u7.md @@ -261,11 +261,112 @@ A sequence is a **function** with a domain of all positive integers in sequence, - The **recursive** formula for a sequence is $t_n = t_{n-1} + 2$ where $t_1 = 1$. - The **arithmetic** formula for a sequence is $t_n = 2n-1$. +If the sequence is infinite, as $n$ becomes very large: + + - If the sequence continuously grows, it **tends to infinity**. (E.g., $a_n = n^2, n ≥ 1$) + - If the sequence gets closer to a real number and converges on it, it **converges to a real limit**, or is convergent**. (E.g., $a_n = \frac{1}{n}, n ≥ 1$) + - If the sequence never approaches a number, it **does not tend to a limit**, or is **divergent**. (E.g., $a_n = \sin(n \pi)$) + ### Limits +A **limit** to a function is the behaviour of that function as a variable approaches, **but does not equal**, another variable. + +!!! example + $$\lim_{x \to c} f(x) = L$$ + "The limit of $f(x)$ as $x$ approaches $c$ is $L$." + +If the lines on both sides of a limit do not converge at the same point, that limit *does not exist*. + +If the lines on both sides of a limit become arbitrarily large as $x$ approaches $a$, it approaches infinity. +$$\lim_{x \to a} f(x) = ∞$$ + +### One-sided limits + +A positive or negative sign is used at the top-right corner of the value approached to denote if that limit applies only to the negative or positive side, respectively. A limit without this sign applies to both sides. + +!!! example + - $\lim_{x \to 3^-} f(x) = 2$ shows that as $x$ approaches $3$ from the negative (usually left) side, $f(x)$ approaches $2$. + - $\lim_{x \to 3^+} f(x) = 2$ shows that as $x$ approaches $3$ from the positive (usually right) side, $f(x)$ approaches $2$. + - $\lim_{x \to 3} f(x) = 2$ shows that as $x$ approaches $3$ from either side, $f(x)$ approaches $2$. + +If $\lim_{x \to c^-} f(x) ≠ \lim_{x \to c^+} f(x)$, $\lim_{x \to c} f(x)$ **does not exist**. + +### Properties of limits + +The following properties assume that $f(x)$ and $g(x)$ have limits at $x = a$, and that $a$, $c$, and $k$ are all real numbers. + + - $\lim_{x \to a} k = k$ + - $\lim_{x \to a} x = a$ + - $\lim_{x \to a} [f(x) ± g(x)] = \lim_{x \to a} f(x) ± \lim_{x \to a} g(x)$ + - $\lim_{x \to a} [f(x) \cdot g(x)] = [\lim_{x \to a} f(x)] [\lim_{x \to a} g(x)]$ + - $\lim_{x \to a} [k \cdot f(x)] = k \cdot \lim_{x \to a} f(x)$ + - $\lim_{x \to a} [f(x)]^2 = [\lim_{x \to a} f(x)]^2$ ### Evaluating limits +When solving for limits, there are five central strategies used, typically in this order if possible: + +#### Direct substitution + +Substitute $x$ as $a$ and solve. + +??? example + $$ + \lim_{x \to 5} (x^2 + 4x + 3) \\ + = 5^2 + 4(5) + 3 \\ + = 48 + $$ + +If **only** direct substitution fails and returns $\frac{0}{0}$, continue on with the following steps. If **only** the denominator is $0$, the limit **does not exist**. + +#### Factorisation, expansion, and simplification + +Attempt to factor out the variable as much as possible so that the result is not $\frac{0}{0}$, and then perform direct substitution. + +??? example + $$ + \lim_{x \to 1} \frac{x^2 - 1}{x-1} \\ + = \lim_{x \to 1} \frac{(x + 1) (x - 1)}{x-1} \\ + = \lim_{x \to 1} (x+1) \\ + = 1 + 1 \\ + = 2 + $$ + +#### Rationalisation + +If there is a square root, multiplying both sides of a fraction by the conjugate may allow direct substitution or factorisation. + +??? example + $$ + \lim_{x \to 0} \frac{\sqrt{1-x}-1}{x} \\ + = \lim_{x \to 0} \frac{\sqrt{1-x}-1}{x} \cdot \frac{\sqrt{1-x}+1}{\sqrt{1-x}+1} \\ + = \lim_{x \to 0} \frac{1-x - 1}{x\sqrt{1-x} + x} \\ + = \frac{1}{\sqrt{1-x} + 1} \\ + = \frac{1}{\sqrt{1-0} + 1} \\ + = \frac{1}{2} + $$ + +#### One-sided limits + +There may only be one-sided limits. In this case, breaking the limit up into its two one-sided limits can confirm if the two-sided limit does not exist when looked at together. + +#### Change in variable + +Substituting a variable in for the variable to be solved and then solving in terms of that variable may help. + +??? example + $$ + \lim_{x \to 0} \frac{x}{(x+1^\frac{1}{3}-1} \\ + \text{let } (x+1)^\frac{1}{3} \text{ be } y \\ + x + 8 = y^3 \\ + x = y^3 - 8, \text{as } x \to 0, y \to 2 \\ + \lim_{y \to 2} \frac{y-2}{y^3 - 8} \\ + = \frac{(y-2)(y^2 + 4y + 4)}{(y^3-8)(y^2 + 4y + 4)} \\ + = \frac{1}{y^2 + 4y + 4} \\ + = \frac{1}{1^2 + 4(1) + 4} \\ + = \frac{1}{12} + $$ + ## Resources - [IB Math Analysis and Approaches Syllabus](/resources/g11/ib-math-syllabus.pdf)