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#1 2017-03-08 17:49:10

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Increasing and decreasing concept

Please help me in this question
Question
Determine the interval in which f is increasing or decreasing for the domain mentioned 
F(x)=sin(x)       x belongs (-pi,pi)

2nd question
And f(x)=cos(x) x belongs to (-pi÷2,pi÷2)


MZk

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#2 2017-03-08 20:55:07

bob bundy
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Registered: 2010-06-20
Posts: 8,139

Re: Increasing and decreasing concept

hi Zeeshan 01

If you draw the graph of a function, increasing means those sections of the graph where the curve is going upwards,  and decreasing means the curve is going downwards.

You may be able to answer these questions just by sketching the graphs.  Or you could differentiate to get the gradient function and then consider: when is the gradient function positive and when is it negative?

There is a really useful grapher at this MathisFun site: http://www.mathsisfun.com/data/function-grapher.php

Bob


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#3 2017-03-09 00:00:57

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

Plese show me method of differntiate


MZk

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#4 2017-03-09 01:09:10

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,139

Re: Increasing and decreasing concept

Differentiation is a huge topic.  Whole books have been written about it!  You can make a start here:

http://www.mathsisfun.com/calculus/deri … ction.html

But you can avoid this by making the graphs.

Bob


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#5 2017-03-09 01:27:45

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

i perform all calculatiions in differntiation but this topic i dont understand


MZk

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#6 2017-03-09 01:42:50

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,139

Re: Increasing and decreasing concept

Here is a graph of a function.  The green curves show where it is increasing.  The red curves show where it is decreasing.

06bDHe5.gif

Bob


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#7 2017-03-09 05:02:16

CIV
Member
Registered: 2014-11-09
Posts: 68

Re: Increasing and decreasing concept

The derivative of sin x is cos x. Derivatives are equations used to calculate the SLOPE of a tangent line at some point on a curve. The curve  is a function such as X^2 or in your example sin x. The derivative of X^2 is 2x. 2x is a linear equation and therefore is refereed to as a SLOPE equation. You can use that slope equation to calculate the ACTUAL slope of a tangent line on the curve of X^2 by choosing a point within the domain of the curve. Lets choose x = 2. The slope would be 2(2) = 4. The slope is represented with the letter m, so m = 4. Now that you have the slope, you need a y value to go with your x value of 2. Use that x value with the original equation to get yourself a y value: (2)^2 = 4. So now you have yourself a the slope of a tangent line at the point (2,4) on the curve X^2. Now what you need is the equation of this tangent line at that point. You remember the point-slope formula? Plug in all the information you have and solve for the equation for that tangent line: y - y2 = m(x-x2); y - 4 = 4(x - 2); y = 4(x - 2) + 4 = 4x - 8 + 4 = 4x - 4. So you have a point (2, 4); a slope equation of f'(x) = 2x; a slope of m = 4; and an equation of a tangent line, y = 4x - 4, at that point.

That's a basic derivative explanation. Forgive me if it appears that I'm assuming you know nothing.

It works the same for the function sin x. The slope equation sin x is f'(x) = cos x. One thing to keep in mind is that the graph of sin x goes to infinity to both direction. It repeats, goes round and round the unit circle. This is why your given an interval of (-pi, pi).

Now, it says determine when f is increasing and decreasing. A straight line is increasing when it's slope is positive and is decreasing when it's slope of negative. So if m = -2, that line is decreasing and if m = 4, that line is increasing. Positive increasing, negative decreasing.

So you have a function sin x and a slope equation for that function, which is cos x. Do you know you know how to find minima and maxima? The slope of minima and maxima is zero, m = 0. So now you know how to find minima and maxima. You set the first derivative, aka the slope equation for the function sin x, to zero. So now that cos x = 0, you have to ask yourself when is cos x zero? Cos x is zero at pi/2 and 3pi/2.

You might be asking why you have to find minima and maxima? By finding minima and maxima, you'll know where to check for increasing and decreasing slopes.

So now that you know that cos x = 0 at pi/2 and 3pi/2, are these values within your interval? pi/2 is, but 3pi/2 is not. If pi/2 is within the interval (-pi, pi), then so is -pi/2.

So you have these two points now. At these points cos x = 0 and being that cos x is the slope equation for sin x... the points on sin x with a slope of zero are the minima and maxima, the high and low points.

Now that you know where the high and low points are on sin x, those being -pi/2 and pi/2, you now know where to look for increasing and decreasing. You look between the high and low points, the minima and maxima. So you need a point between 0 and  pi/2, and pi/2 and pi. Let's chose pi/4 and 3pi/4. If pi/4 and 3pi/4 are in the interval (-pi, pi) then so is -pi/4 and -3pi/4.

Now that we have points between -pi, -pi/2, pi/2, and pi; we can check for increasing and decreasing. Just use your calculator for this. f'(pi/4) = cos (pi/4) = 0.71, is that increasing or decreasing? Its increasing. Now we check f'(3pi/4) = cos (3pi/4) = -0.71, is that increasing or decreasing? It's decreasing. Do that rest and you we see whats happening.

Now remember! SIN X is the FUNCTION or CURVE. COS X is the DERIVATIVE or SLOPE EQUATION for a tangent line touching the function sin x. 0.71 and -0.71 are SLOPES of the tangent lines, the slope being the variable m, at the points pi/4 and 3pi/4.

If you want, you can solve for the equations of the tangent lines that same way I did in the very first explanation. Solve sin x using your x values to get y values. Using those y values, the x values, and the slopes, you can use the point-slope formula to solve for the tangent line equations.

I hope this helps. I can't believe I spent this much time writing all this out....

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#8 2017-03-15 04:08:47

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

I don't understand it well !!!!
Please explain this question
F(x)=sin(x)       x belongs (-pi,pi)


MZk

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#9 2017-03-15 04:10:09

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

How we  get this?????
pi/4 and 3pi/4.


MZk

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#10 2017-06-23 20:25:41

Timo678
Member
Registered: 2017-06-23
Posts: 1

Re: Increasing and decreasing concept

I have no idea of it.

Last edited by Timo678 (2017-09-12 14:36:03)

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#11 2017-06-24 00:36:02

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,139

Re: Increasing and decreasing concept

hi Zeeshan 01

In order to help you I need to know what you understand and what is new to you.

Q1. Most people learn first about measuring angles in degrees.  Later they learn about another way to measure angles : radians.  Do you know about radians?

Q2. Do you know how to make the graph for sin(x)?  Did you understand the graph I showed in post 6 ?

Q3.  I also do not know where " pi/4 and 3pi/4. " came from.  These are not the answers to either of the questions you asked in post 1.  Why do you think they have anything to do with those questions?

Bob


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#12 2017-06-24 05:48:58

iamaditya
Member
From: Planet Mars
Registered: 2016-11-15
Posts: 725

Re: Increasing and decreasing concept

Bob,
Did you become the administrator? Congratulations, bob for your promotion!

Last edited by iamaditya (2017-06-24 05:49:09)


Practice makes a man perfect.
There is no substitute to hard work
All of us do not have equal talents but everybody has equal oppurtunities to build their talents.-APJ Abdul Kalam

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#13 2017-06-24 19:04:56

bob bundy
Administrator
Registered: 2010-06-20
Posts: 8,139

Re: Increasing and decreasing concept

hi iamaditya

Thanks.  Ganesh has also been made an administrator.  Is it a promotion?  I feel it's a huge responsibility.  Let's hope I'm up to the task. 

Bob


Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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#14 2017-06-27 17:10:01

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

aditya wrote:

Bob,Did you become the administrator? Congratulations, bob for your promotion!.

WHAT IS THIS???


MZk

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#15 2017-06-30 02:57:57

Mathegocart
Member
Registered: 2012-04-29
Posts: 1,882

Re: Increasing and decreasing concept

Zeeshan 01 wrote:
aditya wrote:

Bob,Did you become the administrator? Congratulations, bob for your promotion!.

WHAT IS THIS???

It's not an enigma, Zeeshan. Aditya is simply indicating his surprise that Bob is now the Administrator.


The integral of hope is reality.

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#16 2017-06-30 19:47:11

Zeeshan 01
Member
Registered: 2016-07-22
Posts: 648

Re: Increasing and decreasing concept

oK No PRoB!!!


MZk

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