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# fp1 more linear laws Watch

1. This is the question

--------------------------
t = 1 2 3 4 5
m = 24 28.9 34.5 41.5 49.8

It is believed that a formula such as m = ab^t connects m and t.

Find the values of a and b.

-------------------------------------

I've done
ln m = ln a + tlnb

then I've used the scatter graph and regression calculator as follows:

entered x values with the t values
entered y values with ln m values

then I've looked at rg ax+b and got a =0.182 and b = 2.9

The answer in the book is a=20, b= 1.2.

I would appreciate some pointers.

Thanks
2. (Original post by maggiehodgson)
This is the question

--------------------------
t = 1 2 3 4 5
m = 24 28.9 34.5 41.5 49.8

It is believed that a formula such as m = ab^t connects m and t.

Find the values of a and b.

-------------------------------------

I've done
ln m = ln a + tlnb

then I've used the scatter graph and regression calculator as follows:

entered x values with the t values
entered y values with ln m values

then I've looked at rg ax+b and got a =0.182 and b = 2.9

The answer in the book is a=20, b= 1.2.

I would appreciate some pointers.

Thanks

I have just put the values into this regression calculator and I get what the book gets

http://www.easycalculation.com/stati...regression.php

Oh - I see what you have done

you have a and b the wrong way round and you have forgotten to raise e to the power
3. (Original post by TenOfThem)
I have just put the values into this regression calculator and I get what the book gets

http://www.easycalculation.com/stati...regression.php

Oh - I see what you have done

you have a and b the wrong way round and you have forgotten to raise e to the power

Thanks - what with a and b in the question and and b on the calculator I got in a real muddle and then didn't do the final step.
4. (Original post by maggiehodgson)
Thanks - what with a and b in the question and and b on the calculator I got in a real muddle and then didn't do the final step.
easily done
5. (Original post by maggiehodgson)
Thanks - what with a and b in the question and and b on the calculator I got in a real muddle and then didn't do the final step.
So change the letters in the question to something different from a and b, rather than using the same letters to stand for two different quantities (a useful hint for all Maths).
6. (Original post by tiny hobbit)
So change the letters in the question to something different from a and b, rather than using the same letters to stand for two different quantities (a useful hint for all Maths).
Thanks. Yes I did that in order to discover where I went wrong. I will remember that lesson.

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