### Archive

Archive for December, 2010

## Complex Numbers: four rules

Consider complex numbers in the form $a+bi$

Examples

$(2+i)+(4+i)=(2+4)+(1+1)i=6+2i$

$(3+5i)+(-2-3i)=(3-2)+(5-3)i=1+2i$

$(4-i)+(-4+7i)=(4-4)+(-1+7)i=6i$

### Subtracting complex numbers

When subtracting complex numbers we subtract the real part and then we subtract the imaginary part

Examples

$(2+i)-(4+i)=(2-4)+(1-1)i=-2$

$(3+5i)-(-2-3i)=(3+2)+(5+3)i=5+8i$

$(4-i)-(-4+7i)=(4+4)+(-1-7)i=8-8i$

### Multiplying complex numbers

When multiplying complex numbers we need to remember that $i\times i = -1$

Examples

$(2+i)(4+i)=8+2i+4i+i^2=7+6i$

$(3+5i)(-2-3i)=-6-9i-10i-15i^2=9-19i$

$(4-i)(-4+7i)=-16+28i+4i-7i^2=-9+32i$

### Dividing complex numbers

Dividing complex numbers requires us to get rid of the imaginary part from the denominator.

We can do this by multiplying the denominator and numerator by the complex conjugate of the denominator.

Examples

$\displaystyle z=\frac{2+8i}{1+i}$

$\displaystyle \implies z=\frac{2+8i}{1+i}\times \frac{1-i}{1-i}$

$\displaystyle \implies z=\frac{10+6i}{2}=5+3i$

$\displaystyle w=\frac{3-i}{4+i}$

$\displaystyle \implies w=\frac{3-i}{4+i}\times \frac{4-i}{4-i}$

$\displaystyle \implies w=\frac{11-7i}{17}$

$\displaystyle \implies w=\frac{11}{17}- \frac{7}{17}i$

Categories: Complex numbers

## Sketching derivative graphs (cubics)

The important thing to remember here is that we are doing a sketch.  It does not have to be accurate, but id does have to conatian all the key points.

Hints:

1. A cubic differentiates to a quadratic (parabola)
2. A the turning points have a gradient of zero and will move to the x axis on the derivative sketch
3. A point with a positive gradient will be above the x axis in the sketch
4. A point with a pnegative gradient will be below the x axis in the sketch

Examples

Describe and sketch the derivative graph for the function below

The critical points are $x=0\mbox{ and }x=\frac{4}{3}$, because this is where the turning points occur.

These points will be on the x axis in the sketch.

For $x<0$ the gradient is positive.

These points will be above the x axis in the sketch.

For $0 the gradient is negative .

These points will be below the x axis in the sketch.

The graph is a cubic so the sketch will be a parabola

### Practice

Categories: Differentiation