# Definite integration- Evaluate

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#1
Question:
Given that ∫ upper limit 9 lower limit 3 f(x)dx=7, evaluate∫ upper limit 9 lower limit 3 2f(x)+1dx.

As far as I understand
∫ upper limit 9 lower limit 3 f(x)dx = F(9)-F(3)
So, F(9)-F(3)=7

If I knew the f(x) I could solve it easy enough.
If d/dx F(x)=f(x)

Help in the right direction would be grateful.

Thank you
0
4 weeks ago
#2
(Original post by KingRich)
Question:
Given that ∫ upper limit 9 lower limit 3 f(x)dx=7, evaluate∫ upper limit 9 lower limit 3 2f(x)+1dx.

As far as I understand
∫ upper limit 9 lower limit 3 f(x)dx = F(9)-F(3)
So, F(9)-F(3)=7

If I knew the f(x) I could solve it easy enough.
If d/dx F(x)=f(x)

Help in the right direction would be grateful.

Thank you
A rough sketch of f(x) might help. Keep it simple, you can choose what the "curve" looks like. The area under it (between the curve and the x-axis) is 7.
Then double f(x) and add 1 to 2f(x), doing another sketch of 2f(x)+1. How will (the area under) the curve change?

While its simple to do algebraically, having a proper insight is useful.
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#3
(Original post by mqb2766)
A rough sketch of f(x) might help. Keep it simple, you can choose what the "curve" looks like. The area under it (between the curve and the x-axis) is 7.
Then double f(x) and add 1 to 2f(x), doing another sketch of 2f(x)+1. How will (the area under) the curve change?

While its simple to do algebraically, having a proper insight is useful.
I did try to visualise as you said but if what I believe you mean by double f(x) is, then I end up with f(6)-f(18).

Erm, am I right that a transformation in the graph here is happening? For example the graph is vertically stretching by 2?
0
4 weeks ago
#4
(Original post by KingRich)
I did try to visualise as you said but if what I believe you mean by double f(x) is, then I end up with f(6)-f(18).

Erm, am I right that a transformation in the graph here is happening? For example the graph is vertically stretching by 2?
First paragraph - thats incorrect, its not a horizontal (x-axis) scaling.
Second paragraph - thats right, its a vertical scaling (y-axis) by 2 and add 1.

Tbh, I'd change the question slightly. If the definite integral is 6 (rather than 7) between x=3 and x=9, what is the simplest function f(x) that you could imagine which has that area between those limits. Then multiply it (y-axis) by 2 and add 1. What is the new area?

Then it should be easy to work out if the original definite integral is 7.
Last edited by mqb2766; 4 weeks ago
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#5
(Original post by mqb2766)
First paragraph - thats incorrect, its not a horizontal (x-axis) scaling.
Second paragraph - thats right, its a vertical scaling (y-axis) by 2 and add 1.

Tbh, I'd change the question slightly. If the definite integral is 6 (rather than 7) between x=3 and x=9, what is the simplest function f(x) that you could imagine which has that area between those limits. Then multiply it (y-axis) by 2 and add 1. What is the new area?

Then it should be easy to work out if the original definite integral is 7.
Sorry, I am so lost right now. I might have to take a break and come back
0
4 weeks ago
#6
(Original post by KingRich)
Sorry, I am so lost right now. I might have to take a break and come back
You may be overthinking it a bit. Consider
f(x) = 1
What is the area of the (rectangle) between x=3 and x=9 (definite integral)?

Then what is the area under function g(x) (between it and the x-axis) where
g(x) = 2f(x) + 1 = f(x) + f(x) + 1 = ...
again between x=3 and x=9.

Try sketching what you think and upload?
Last edited by mqb2766; 4 weeks ago
0
#7
(Original post by mqb2766)
You may be overthinking it a bit. Consider
f(x) = 1
What is the area of the (rectangle) between x=3 and x=9 (definite integral)?

Then what is the area under function g(x) (between it and the x-axis) where
g(x) = 2f(x) + 1 = f(x) + f(x) + 1 = ...
again between x=3 and x=9.

Try sketching what you think and upload?
I believe our initial approach on this was wrong.

The 1 is yet to be integrated. The example given tells us that between the limits 9 and 3 our y=7 and upon integration of 1 between those limits you find the value of 6.

So, 2f(x)=14 + 6= 20

I think the way that it has been written is misleading, or maybe a lack of understanding has confused me.
I realised now a transformation cannot happen between two points only. If there’s a transformation of any value it effects the whole graph.
0
4 weeks ago
#8
(Original post by KingRich)
I believe our initial approach on this was wrong.

The 1 is yet to be integrated. The example given tells us that between the limits 9 and 3 our y=7 and upon integration of 1 between those limits you find the value of 6.

So, 2f(x)=14 + 6= 20

I think the way that it has been written is misleading, or maybe a lack of understanding has confused me.
I realised now a transformation cannot happen between two points only. If there’s a transformation of any value it effects the whole graph.
That looks good. You're simply scaling and translating the y axis so the area under it has a similar transformation
2f(x)
is
2*area under f()
2f(x) + 1
means you add a strip of height 1 and width 6 (9-3) to the function, so the area is increased by 6.

As you say but think about the transformation to the function and hence the area under it.
Last edited by mqb2766; 4 weeks ago
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