Slopes and Areas

• Frequently we will want to know the slope of a curve at some point.

• Or an area under a curve.

We calculate area under a curve as the sum of areas of many rectangles under the curve.

Review: Axes• When two things vary, it helps to draw a

picture with two perpendicular axes to show what they do. Here are some examples:

y

x

x

t

y varies with x x varies with t

Here we say “ y is a function of x” . Here we say “x is a function of t” .

Positions

• We identify places with numbers on the axes

The axes are number lines that are perpendicular to each other.Positive x to the right of the origin (x=0, y=0), positive y above the origin.

Straight Lines

• Sometimes we can write an equation for how one variable varies with the other. For example a straight line can be described as

y = ax + b Here, y is a position on the line along the y-axis, x is a

position on the line along the x- axis, a is the slope, and b is the place where the line hits the y-axis

Straight Line Slope

y = ax + b The slope, a is just the rise y divided by the run x. We can do this anywhere on the line.

y means y finish – y start, here 0 - 3 = -3

x means x finish – x start, here 2 - 0 = +2

So the slope of the line here

is y = -3

x 2Remember: Rise over Run and up and right are positive

Or, proceed in the positive x direction for some number of units, and count the number of units up or down the y changes

y- intercept

y = ax + b The intercept b is y = +3 when x = 0 for this line

Equation for this line

y = ax + b

So the equation of the line

here is y = -3 x + 3

2

Equation of Example Line

An example: a flow gauge on a small creek

• Suppose we plot as the vertical axis the flow rate in m3/ hour and the horizontal axis as the time in hours Then the line tells us that a

cloudburst caused the creek to flow at 3 m3/hour initially, but always decreased at a rate (slope) of - 3/2 m3 per hour after that, so it stopped after two hours. The area under the line is the total volume of water the flowed past the gauge during the two hours. A = 1/2bh = 1/2 x 2 x 3 = 3 m3

This plot, flow vs. time, is a hydrograph. The area under the curve is the volume of runoff.

Trig• Perpendicular axes and lines are very handy. Recall we said we use

them for vectors such as velocity. To break a vector into components, we use trig. The sine of angle theta is r times the vertical (rise) part of this triangle, and the cosine of angle is r times the horizontal (run) .

This vector with size r and direction , has been broken down into components. Along the y-axis, the rise is y = +r sin Along the x-axis, the run is x = +r cos

Demo: the sine is the ordinate (rise) divided by the hypotenusesin = rise / r so the rise = r sin

Similarly the run = r cos

hypo

tenus

e

rise

run

Okay, sines and cosines, but what’s a Tangent?

A Tangent Line is a line that is going in the direction of a point proceeding along the curve.

A Tangent at a point is the slope of the curve there.

A tangent of an angle is the sine divided by the cosine.

Tangents to curves• Here the vector r shows the velocity of a particle moving along the blue line f(x)• At point P, the particle has speed r and the direction shown makes an angle to

the x-axis

slope = f(x + h) –f(x) (x + h) – x This is rise over run as always

Lets see that is r sin tan r cos

P

The slope, and by extension the accurate derivative with h very small, is a tangent to the curve.

Slope at some point on a curve• We can learn the same things from any curve if we have an equation for

it. We say y = some function f of x, written y = f(x). Lets look at the small interval between x and x+h. y is different for these two values of x.

The slope is rise over run as always

slope = f(x + h) –f(x) (x + h) – x

rise

derivative dy/dx = f(x + h) –f(x) lim h=>0 h

The exact slope at some point on the curve is found by making the distance between x and x+h small, by making h really small

This is inaccurate for a point on a curve, because the slope varies.run

A simple derivative for Polynomials• The derivative of f(x)f’(x) = f(x + h) – f(x) = f(x + h) – f(x) lim h=>0 (x + h) – x lim h=>0 h

is known for all of the types of functions we will use in Hydrology.

For example, suppose y = xn

where n is some constant and x is a variableThen dy/dx = nxn-1

dy/dx means “The change in y with respect to x”

Some Examples for Polynomials• (1) Suppose y = x4 . What is dy/dx?

dy/dx = 4x3

• (2) Suppose y = x-2

What is dy/dx? dy/dx = -2x-3

For polynomials y = xn dy/dx = nxn - 1

Differentials• Those new symbols dy/dx mean the really

accurate slope of the function y = f(x) at any point. We say they are algebraic, meaning dx and dy behave like any other variable you manipulated in algebra class.

• The small change in y at some point on the function (written dy) is a separate entity from dx.

• For example, if y = xn

• dy/dx = nxn-I also means dy = nxn-I dx

Variable names

• There is nothing special about the letters we use except to remind us of the axes in our coordinate system

• For example, if y = un

• dy = nun-I du is the same as the previous formula.

y = un

u

Constants Alone• The derivative of a constant is zero.• If y = 17, dy/dx = 0 because constants don’t

change, and the constant line has zero slope

Y = 1717

y

x

X alone

• Suppose y = x What is dy/dx?

• Y = x means y = x1. Just follow the rule.

• Rule: if y = xn then dy/dx = nxn – 1

• So if y = x , dy/dx = 1x0 = 1

• Anything to the power zero is one.

A Constant times a Polynomial

• Suppose y = 4 x7 What is dy/dx?

• The derivative of a constant times a polynomial is just the constant times the derivative of the polynomial.

• So if y = 4 x7 , dy/dx = 4 ( 7x6)

Multiple Terms in a sum

• The derivative of a function with more than one term is the sum of the individual derivatives.

• If y = 3 + 2t + t2 then dy/dt = 0 + 2 +2t

• Notice 2t1 = 2t

For polynomials y = xn dy/dx = nxn - 1

The derivative of a product

• In words, the derivative of a product of two terms is the first term times the derivative of the second, plus the second term times the derivative of the first.

Exponents

• aman = am+n am/an = am-n

• (am)n = amn (ab)m = ambm

• (a/b)m = am/bm a-n = 1/an

Suppose m and n are rational numbers

You can remember all of these just by experimentingFor example 22 = 2x2 and 24= 2x2x2x2 so 22x24 = 2x2x2x2x2x2 = 26

reminds you of rule 1Rule 6, a-n = 1/an , is especially useful

Logarithms

• Logarithms (Logs) are just exponents

• if by = x then y = logb x

e• e is a base, the base of the so-called natural

logarithms.• It has a very interesting derivative.• Suppose u is some function• Then d(eu) = eu du• Example: If y = e2x what is dy/dx?• here u = 2x, so du = 2• Therefore dy/dx = e2x . 2

Integrals

• The area under a function between two values of, for example, the horizontal axis is called the integral. It is a sum of a series of very small rectangles, and is indicated by a very tall and thin script S, like this:

•

Integrals

• To get accuracy with areas we use extremely thin rectangles, much thinner than this.

Example 1

• If y=3x5 Then dy/dx = 15x4

• Then y = 15x4 dx = 3x5 + a constant

Integration is the inverse operation for differentiation

We have to add the constant as a reminder because, if a constant was present in the original function, it’s derivative would be zero and we wouldn’t see it.

Example2: a trickSometimes we must multiply by one to get a known integral form. For example, we know:

A useful method• When a function changes from having a

negative slope to a positive slope, or vs. versa, the derivative goes briefly through zero.

• We can find those places by calculating the derivative and setting it to zero.

Getting useful numbers• Suppose y = x2. • (a) Find the minimum If y = x2 then dy/dx = 2x1 = 2x. Set this equal to zero 2x=0 so x=0 y = x2 so if x = 0 then y = 0 Therefore the curve has zero slope at (0,0)

Getting useful numbers

• Suppose y = x2. • (b) Find the slope at x=3(a) If y = x2 then dy/dx = 2x1 = 2x. Set x=3 then the slope is 2x = 2 . 3 = 6

Getting useful numbers• Here is a graph of y = x2

• Notice the slope is zero at (0,0)• The slope at (x=3,y=9) is +6/1 = 6