 July 17, 2019, 10:20:25 am

### AuthorTopic: VCE Specialist 3/4 Question Thread!  (Read 1061217 times) Tweet Share

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#### Ansaki

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« Reply #9495 on: June 13, 2019, 10:43:57 pm »
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hey guys i need help with a question, thanks!
question 1:  Evaluate   $\int_{π/2}^{π} sin^{3}xcos^{2}x\,dx$
btw my first time using the latex system and it was sooo difficult lol

#### undefined

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« Reply #9496 on: June 13, 2019, 11:18:50 pm »
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hey guys i need help with a question, thanks!
question 1:  Evaluate   $\int_{π/2}^{π} sin^{3}xcos^{2}x\,dx$
btw my first time using the latex system and it was sooo difficult lol

Split Sin^3(x) into Sin(x) Sin^2(x) then use the pythagorean identity to turn sin^2(x) into 1-cos^2(x). Let u=cos(x) which cancels out the remaining sin(x) and you should get an easy to evaluate integral.
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#### AlphaZero ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9497 on: June 13, 2019, 11:23:18 pm »
+1
hey guys i need help with a question, thanks!
question 1:  Evaluate   $\int_{\pi/2}^\pi \sin^3(x)\cos^{2}(x)\,dx$
btw my first time using the latex system and it was sooo difficult lol

I edited your LaTeX code a bit to make it look nicer. It takes time to learn, but don't get hung up on it. It's more important that you understand maths first Here's a starting hint: $\int_{\pi/2}^\pi \sin^3(x)\cos^2(x)\,dx=\int_{\pi/2}^\pi \sin(x)\big(1-\cos^2(x)\big)\cos^2(x)\,dx=\int_{\pi/2}^\pi \sin(x)\big(\!\cos^2(x)-\cos^4(x)\big)dx$ What substitution could you make?

Edit: undefined beat me to it lol
« Last Edit: June 13, 2019, 11:24:54 pm by AlphaZero »
2015$-$2017:  VCE
2018$-$2021:  Bachelor of Biomedicine and Concurrent Diploma in Mathematical Sciences, University of Melbourne

#### Ansaki

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« Reply #9498 on: June 16, 2019, 03:47:10 pm »
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I edited your LaTeX code a bit to make it look nicer. It takes time to learn, but don't get hung up on it. It's more important that you understand maths first Here's a starting hint: $\int_{\pi/2}^\pi \sin^3(x)\cos^2(x)\,dx=\int_{\pi/2}^\pi \sin(x)\big(1-\cos^2(x)\big)\cos^2(x)\,dx=\int_{\pi/2}^\pi \sin(x)\big(\!\cos^2(x)-\cos^4(x)\big)dx$ What substitution could you make?

Edit: undefined beat me to it lol

Split Sin^3(x) into Sin(x) Sin^2(x) then use the pythagorean identity to turn sin^2(x) into 1-cos^2(x). Let u=cos(x) which cancels out the remaining sin(x) and you should get an easy to evaluate integral.

hey guys! sorry for the late reply but thanks so much helped me out massively!

#### Tatlidil ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9499 on: June 16, 2019, 07:11:05 pm »
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Whats up AN!
So I have a few questions...(I have a SAC tomorrow, part 1 is vectors/vector calc)
How do I find linear dependency/independency
How do I find perpendicular vectors
Let me know if you need a question Goal:
2019 - Specialist , Physics 
ATAR: 99+

#### DBA-144

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« Reply #9500 on: June 16, 2019, 08:17:55 pm »
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Whats up AN!
So I have a few questions...(I have a SAC tomorrow, part 1 is vectors/vector calc)
How do I find linear dependency/independency
How do I find perpendicular vectors
Let me know if you need a question Linear dependency just use the definition A (m) + B (n) = C where A, B and C are vectors as I don't know latex. the definition should be in your textbook. Essentially, you are just showing that you can multiply the two vectors (not necessarily by the same factor) and add them to form the third vector.

In 2 dimensions, 3 vectors will always be linearly dependent and for the 3 dimensions, 4 vectors will always be linearly dependent. We need to use the above definitions when we can't be sure and need to check.

The dot product is a.b= |a| |b| cos theta. Perpendicular --> 90 degree angle at intersection --> a.b = |a| |b| x 0 = 0. that is, perpendicular vectors are such that a.b=0. THe idea here is that cos 90=0, by the way.

Are there any questions specifically that you had?

#### Tatlidil ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9501 on: June 16, 2019, 10:44:14 pm »
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Linear dependency just use the definition A (m) + B (n) = C where A, B and C are vectors as I don't know latex. the definition should be in your textbook. Essentially, you are just showing that you can multiply the two vectors (not necessarily by the same factor) and add them to form the third vector.

In 2 dimensions, 3 vectors will always be linearly dependent and for the 3 dimensions, 4 vectors will always be linearly dependent. We need to use the above definitions when we can't be sure and need to check.

The dot product is a.b= |a| |b| cos theta. Perpendicular --> 90 degree angle at intersection --> a.b = |a| |b| x 0 = 0. that is, perpendicular vectors are such that a.b=0. THe idea here is that cos 90=0, by the way.

Are there any questions specifically that you had?
How do you show independency?

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#### S_R_K

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« Reply #9502 on: June 17, 2019, 05:46:45 pm »
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How do you show independency?

To show that vectors a, b, c are linearly independent, show that it is not possible to find real numbers m, n such that c = ma + nb. This will generally involve finding a set of simultaneous equations in m and n, and showing that there is no simultaneous solution.

#### AlphaZero ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9503 on: June 18, 2019, 08:38:12 pm »
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To show that vectors a, b, c are linearly independent, show that it is not possible to find real numbers m, n such that c = ma + nb. This will generally involve finding a set of simultaneous equations in m and n, and showing that there is no simultaneous solution.

I'm in the middle of exams right now, so I'll keep this brief, but this actually is not true and is a common misconception among Specialist Maths students. I'll be writing a concise guide on linear dependence/independence in the near future.

In a nutshell though, the correct statement relating linear combinations of 3 vectors $(\mathbf{a},\mathbf{b},\mathbf{c})$ and linear independence/dependence is: $\exists \,m,n\in\mathbb{R}\setminus\{0\}\ \ \text{s.t.}\ \ \mathbf{c}=m\mathbf{a}+n\mathbf{b}\ \implies\ \mathbf{a},\mathbf{b},\mathbf{c}\ \text{are linearly dependent vectors}$ Simply negating both sides of the statement doesn't make it equivalent. The following is an incorrect statement: $\forall\,m,n\in\mathbb{R}\setminus\{0\},\ \ \mathbf{c}\neq m\mathbf{a}+n\mathbf{b}\ \implies\ \mathbf{a},\mathbf{b},\mathbf{c}\ \text{are linearly independent vectors}$A simple counterexample is shown by the vectors $\mathbf{a}=2\mathbf{i}-\mathbf{j}+\mathbf{k},\ \ \mathbf{b}=-4\mathbf{i}+2\mathbf{j}-2\mathbf{k},\ \ \mathbf{c}=3\mathbf{i}+2\mathbf{j}+\mathbf{k}$ It can very easily be shown that  $\mathbf{c}\neq m\mathbf{a}+n\mathbf{b}\ \ \forall\,m,n\in\mathbb{R}$  yet its obvious that  $\mathbf{a},\mathbf{b},\mathbf{c}$  are linearly dependent since $2\mathbf{a}+\mathbf{b}+0\mathbf{c}=\mathbf{0}.$
2015$-$2017:  VCE
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#### S_R_K

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« Reply #9504 on: June 19, 2019, 06:07:20 pm »
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Thanks AlphaZero, fair enough. I should have said that a set of vectors are linearly independent "if, for each vector in the set, it is not possible to write it as a linear combination (where not all scalars are zero) of the others" without choosing a specific linear combination. In the example you've given, while it's not possible to write c as a linear combination of a and b, it is possible to write it a as a linear combination of b and c -> hence, linearly dependent.

#### AlphaZero ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9505 on: June 19, 2019, 08:47:20 pm »
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Thanks AlphaZero, fair enough. I should have said that a set of vectors are linearly independent "if, for each vector in the set, it is not possible to write it as a linear combination (where not all scalars are zero) of the others" without choosing a specific linear combination. In the example you've given, while it's not possible to write c as a linear combination of a and b, it is possible to write it a as a linear combination of b and c -> hence, linearly dependent.

Indeed, but I think it's very tedious showing that none of the vectors can be written as a linear combination of the rest. For a set of 3 vectors, that would involve solving simultaneous equations 3 times.

To prove linear independence, it's probably best to go back to a definition. Guide coming soon 2015$-$2017:  VCE
2018$-$2021:  Bachelor of Biomedicine and Concurrent Diploma in Mathematical Sciences, University of Melbourne

#### Antae

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« Reply #9506 on: July 01, 2019, 08:14:50 pm »
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For applications of differential equations, do we need to know the logistic equation and will we be required to memorise formulas such as Newton's Law of Cooling or will they be given to us?

Edit: I just realised that Newton's Law of Cooling is not particularly difficult to memorise but what about other equations?
« Last Edit: July 01, 2019, 08:45:49 pm by Antae »

#### Sine

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« Reply #9507 on: July 01, 2019, 09:10:11 pm »
+1
For applications of differential equations, do we need to know the logistic equation and will we be required to memorise formulas such as Newton's Law of Cooling or will they be given to us?

Edit: I just realised that Newton's Law of Cooling is not particularly difficult to memorise but what about other equations?
I think Newton's law of cooling was on the very old study designs formula sheets - as for your question, no, you won't need to memorise any specific differential equation - they will give you what is required.
If you want to make sure of this - ask your teacher or have a look at the study design.

#### AlphaZero ##### Re: VCE Specialist 3/4 Question Thread!
« Reply #9508 on: July 01, 2019, 09:22:55 pm »
+1
For applications of differential equations, do we need to know the logistic equation and will we be required to memorise formulas such as Newton's Law of Cooling or will they be given to us?

Edit: I just realised that Newton's Law of Cooling is not particularly difficult to memorise but what about other equations?

You do not need to memorise any differential equations for specific types of models (eg: Newton's Law of Cooling, the Logistic equation, etc.).

You do however need to be able to read a description of a model and form a differential equation from the information given (eg: in mixing problems, etc.

Otherwise, you'll be given the differential equation (eg: in beam theory problems)
2015$-$2017:  VCE
2018$-$2021:  Bachelor of Biomedicine and Concurrent Diploma in Mathematical Sciences, University of Melbourne

#### Antae

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« Reply #9509 on: July 02, 2019, 09:54:53 pm »
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You do not need to memorise any differential equations for specific types of models (eg: Newton's Law of Cooling, the Logistic equation, etc.).

You do however need to be able to read a description of a model and form a differential equation from the information given (eg: in mixing problems, etc.

Otherwise, you'll be given the differential equation (eg: in beam theory problems)
I think Newton's law of cooling was on the very old study designs formula sheets - as for your question, no, you won't need to memorise any specific differential equation - they will give you what is required.
If you want to make sure of this - ask your teacher or have a look at the study design.

Thanks 