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Writing expressions??! How? (3 marks)

Can someone please explain, what to think/ do when writing an expression?

For example for this question. :smile:

I have wrote
Ali
3 more
2 hours less
72 components

Beth
72 components

How do I write the expression. (part i)
And then please can you go through the rest of the question.

Once I understand what to think, when writing an expression and learn how to do the other parts, it will be a lot more simple!


Question attached

question a.png

Thanks :biggrin:

Scroll to see replies

Reply 1
Avatar for sputum
sputum
9
Ali makes 72 components in t hours
to show how many he makes in 1 hour divide by t
ie 72/t components in t/t = 1 hour
Beth makes 72 components in t+2 hours
divide by t+2

that should get you started:smile:
(edited 11 years ago)
Reply 2
Avatar for TenOfThem
TenOfThem
18
(i) Number per hour = Number/time

Na = 72/t


For Nb you need to change t to t+2 as Beth takes 2 hours longer


Then Nb + 3 = Na and re-arrange
Reply 3
Avatar for ztibor
ztibor
10
Original post by blueray
Can someone please explain, what to think/ do when writing an expression?

For example for this question. :smile:

I have wrote
Ali
3 more
2 hours less
72 components

Beth
72 components

How do I write the expression. (part i)
And then please can you go through the rest of the question.

Once I understand what to think, when writing an expression and learn how to do the other parts, it will be a lot more simple!


Question attached

question a.png

Thanks :biggrin:


i)
t the time for Ali to make 72
then the component per hour
72/t
For Beth
the components per hour
72/t -3 (3 less per hour)
It has all been explained this question... spo I hope that is enough for you.
Original post by blueray
Can someone please explain, what to think/ do when writing an expression?

For example for this question. :smile:

I have wrote
Ali
3 more
2 hours less
72 components

Beth
72 components

How do I write the expression. (part i)
And then please can you go through the rest of the question.

Once I understand what to think, when writing an expression and learn how to do the other parts, it will be a lot more simple!


Question attached

question a.png

Thanks :biggrin:
Reply 5
Avatar for TenOfThem
TenOfThem
18
Original post by ztibor
i)
t the time for Ali to make 72
then the component per hour
72/t
For Beth
the components per hour
72/t -3 (3 less per hour)


Beth takes longer
Reply 6
Avatar for User570431
User570431
OP
15
Original post by sputum
Ali makes 72 components in t hours
to show how many he makes in 1 hour divide by t
ie 72/t components in t/t = 1 hour
Beth makes 72 components in t+2 hours
divide by t+2

that should get you started:smile:



Original post by TenOfThem
(i) Number per hour = Number/time

Na = 72/t


For Nb you need to change t to t+2 as Beth takes 2 hours longer


Then Nb + 3 = Na and re-arrange



Original post by ztibor
i)
t the time for Ali to make 72
then the component per hour
72/t
For Beth
the components per hour
72/t -3 (3 less per hour)



Original post by GreenLantern1
It has all been explained this question... spo I hope that is enough for you.


Why would ali be 72/t

why not 72/t-2 Because it takes him 2 hours less???

Where am I wrong? Please explain.
Reply 7
Avatar for TenOfThem
TenOfThem
18
Original post by blueray
Why would ali be 72/t

why not 72/t-2 Because it takes him 2 hours less???

Where am I wrong? Please explain.


t was defined in the question as the time that Ali takes
Reply 8
Avatar for User570431
User570431
OP
15
Original post by TenOfThem
t was defined in the question as the time that Ali takes


Ok :smile: So when would I be correct then, what would the question have to say, for my answer to be correct?
Reply 9
Avatar for TenOfThem
TenOfThem
18
Original post by blueray
Ok :smile: So when would I be correct then, what would the question have to say, for my answer to be correct?


if t had been defined as the time that Beth took
Reply 10
Avatar for User570431
User570431
OP
15
Original post by sputum
Ali makes 72 components in t hours
to show how many he makes in 1 hour divide by t
ie 72/t components in t/t = 1 hour
Beth makes 72 components in t+2 hours
divide by t+2

that should get you started:smile:



Original post by ztibor
i)
t the time for Ali to make 72
then the component per hour
72/t
For Beth
the components per hour
72/t -3 (3 less per hour)



Original post by GreenLantern1
It has all been explained this question... spo I hope that is enough for you.



Original post by TenOfThem
Beth takes longer


Hence derive the equation 3t(t + 2) = 144.

What does derive mean? How do I do it?
I gave you 3 equations in post 3

combine them

re-arrange
Original post by blueray
?

Right, so the number of components that Ali makes per hour is 72/t
The number of components that Beth makes per hour is 72/t+2.

Ali makes 3 more per hour, so 72t=72t+2+3\frac{72}{t} = \frac{72}{t+2}+3
So, 72t−72t+2=3\frac{72}{t}-\frac{72}{t+2}=3

Now multiply all the numerators by (t) and (t+2) to get:

72(t)(t+2)t−72(t)(t+2)t+2=3(t)(t+2)\frac{72(t)(t+2)}{t}-\frac{72(t)(t+2)}{t+2}=3(t)(t+2)

This is equalise same as: 72(t+2)−72(t)=3(t)(t+2)72(t+2) - 72(t) = 3(t)(t+2)
Multiply out your brackets to get
72t+144−72t=3t(t+2)72t+144-72t = 3t(t+2)
144=3t(t+2)144=3t(t+2)

This help?

EDIT: to figure out part iii) multiply out your brackets in the last equation, and rearrange to get a quadratic, then figure out your values of t from there :smile:
(edited 11 years ago)
Reply 13
Avatar for User570431
User570431
OP
15
Original post by CharlieBoardman
Right, so the number of components that Ali makes per hour is 72/t
The number of components that Beth makes per hour is 72/t+2.

Ali makes 3 more per hour, so 72t=72t+2+3\frac{72}{t} = \frac{72}{t+2}+3
So, 72t−72t+2=3\frac{72}{t}-\frac{72}{t+2}=3

Now multiply all the numerators by (t) and (t+2) to get:

72(t)(t+2)t−72(t)(t+2)t+2=3(t)(t+2)\frac{72(t)(t+2)}{t}-\frac{72(t)(t+2)}{t+2}=3(t)(t+2)

This is equalise same as: 72(t+2)−72(t)=3(t)(t+2)72(t+2) - 72(t) = 3(t)(t+2)
Multiply out your brackets to get
72t+144−72t=3t(t+2)72t+144-72t = 3t(t+2)
144=3t(t+2)144=3t(t+2)

This help?


All the steps are well done thanks :biggrin:
But thing I can't get my head round is
if Ali makes 3 MORE then why is +3 with Beth??
Shouldn't it be
72/t +3 = 72/t+2 ??
Then 72/t - 72/t+2 = -3??????/
Original post by blueray
All the steps are well done thanks :biggrin:
But thing I can't get my head round is
if Ali makes 3 MORE then why is +3 with Beth??
Shouldn't it be
72/t +3 = 72/t+2 ??
Then 72/t - 72/t+2 = -3??????/

I see where your confusion comes from :smile: look at it this way.

Components of Ali per hour = 72/t
Components of Beth per hour = 72/t+2 yes?
Right, so Ali makes 3 more than Beth per hour, so 72/t is 3 more than 72/t+2. So to make them equal, you need to add 3 onto 72/t+2, giving you the 72/t=(72/t+2)+3

This explain it ok? :smile:
Reply 15
Avatar for User570431
User570431
OP
15
Original post by CharlieBoardman
I see where your confusion comes from :smile: look at it this way.

Components of Ali per hour = 72/t
Components of Beth per hour = 72/t+2 yes?
Right, so Ali makes 3 more than Beth per hour, so 72/t is 3 more than 72/t+2. So to make them equal, you need to add 3 onto 72/t+2, giving you the 72/t=(72/t+2)+3

This explain it ok? :smile:


Yeah that makes sense, but why would you want to make them equal? Sorry if I sound silly, I just want to be crystal clear :biggrin:
Ps I have lots of English literature notes etc, if you want :biggrin:
Original post by blueray
Yeah that makes sense, but why would you want to make them equal? Sorry if I sound silly, I just want to be crystal clear :biggrin:
Ps I have lots of English literature notes etc, if you want :biggrin:

Just so that you have something to rearrange and work with. When solving something, you generally want 2 equations which are equal to each other, but layed out differently, so that you can rearrange the, to suit the question.

For example. We used 72/t=(72/t+2)+3, which we could rearrange to get us somewhere.
72/t=72/t or 72/t+2=72/t+2 would have been useless to us.

This ok?

What books are you studying for Lit?
Original post by blueray
Yeah that makes sense, but why would you want to make them equal?



How would you end up with an equation if you did not have 2 things that were equal to each other
Original post by TenOfThem
How would you end up with an equation if you did not have 2 things that were equal to each other

This is precisely the phrase I was looking for! But I ended up rambling on. :curious:
Reply 19
Avatar for User570431
User570431
OP
15
Original post by CharlieBoardman
I see where your confusion comes from :smile: look at it this way.

Components of Ali per hour = 72/t
Components of Beth per hour = 72/t+2 yes?
Right, so Ali makes 3 more than Beth per hour, so 72/t is 3 more than 72/t+2. So to make them equal, you need to add 3 onto 72/t+2, giving you the 72/t=(72/t+2)+3

This explain it ok? :smile:


If it's 3 more isn't it times 3?As you have 3 more lots?

Original post by CharlieBoardman

What books are you studying for Lit?

Inspector calls and of mice and men. :smile:

Original post by TenOfThem
How would you end up with an equation if you did not have 2 things that were equal to each other


Can you help explain charlies point in another way? If I have 2 explanations It would be easier to understand. :biggrin:

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