Past Paper Help? (1 Viewer)

[Zokunu]

ok, what am i doing wrong here?

I'm asked to rearrange this to make k the subject?

T = 2pi√m/k
T^2 = 4pi^2 m/k <--Divide on this by K?
T^2/k = 4pi^2 m
KT^2 = 4pi^2 m
k = 4pi^2m/T <- I got this ==''

is this right?

 

[asianese]

Your 2nd step is incorrect. You have divided by k instead of multiplying.

 

[Zokunu]

It's correct

But the answer shows k = 4pi^2m/KT^2 ???

 

[Menomaths]

Apparently I got the same answer

 

[Zokunu]

Your 2nd step is incorrect. You have divided by k instead of multiplying.

Oh w8 yep my bad.

A plane flies 1600km in 2 hours and 45 minutes. Calculate it's speed correct to 3 significant figures?

S=D/T?

= 1600/2.45? right?

 

[Zokunu]

Meno...which one? this one? k = 4pi^2m/KT^2

 

[Menomaths]

Oh w8 yep my bad.

A plane flies 1600km in 2 hours and 45 minutes. Calculate it's speed correct to 3 significant figures?

S=D/T?

= 1600/2.45? right?

Nope, 1600/ 2.75
The T squared one is correct

 

[Zokunu]

Nope, 1600/ 2.75
The T squared one is correct

kk, thx man

 

[Zokunu]

Bump... How do i factorise these 2?

4x^2 + 17xy + 15y^2

15a^4b - 10a^4b^2 - 20a^3b^3?

thx

 

[MrBeefJerky]

1) 4x^2+17xy+15y^2
Using cross method, you get (4x+5y)(x+3y)
2) 5a^3b(3a-2ab-4b^2)?

 

[Zokunu]

1) 4x^2+17xy+15y^2
Using cross method, you get (4x+5y)(x+3y)
2) 5a^3b(3a-2ab-4b^2)?

thx but can you explain it more simpler? I'm not used to cross method

 

[MrBeefJerky]

It is a bit difficult to explain, but I'll try. You basically draw a cross (X) and on the LHS (end points) you find two values that multiply together to get 4x^2. This could be 4x and x or 2x and 2x. On the RHS (end points) you need two values that multiply to get 15y^2 which could be 5y and 3y or 15y and y. Multiply the two values that are diagonally across and add it with the multiplication of the other diagonal. This should add up to 17xy. If it does then the factors of the equation will be the two values that are opposite each other added together.
4x 5y
X
x 3y
(4x multiply by x) gives 4x^2 (5y multiply by 3y) gives 15y^2 and adding the multiplication of the diagonals is: (4x x 3y) + (x x 5y) = 17xy. Therefore the factors are (4x+5y)(x+3y)

 

[Zokunu]

It is a bit difficult to explain, but I'll try. You basically draw a cross (X) and on the LHS (end points) you find two values that multiply together to get 4x^2. This could be 4x and x or 2x and 2x. On the RHS (end points) you need two values that multiply to get 15y^2 which could be 5y and 3y or 15y and y. Multiply the two values that are diagonally across and add it with the multiplication of the other diagonal. This should add up to 17xy. If it does then the factors of the equation will be the two values that are opposite each other added together.
4x 5y
X
x 3y
(4x multiply by x) gives 4x^2 (5y multiply by 3y) gives 15y^2 and adding the multiplication of the diagonals is: (4x x 3y) + (x x 5y) = 17xy. Therefore the factors are (4x+5y)(x+3y)

Far, thx man I get it now

 

[Hypem]

Far, thx man I get it now

You could also use PSF (product, sum, factors)

4x^2 + 17xy + 15y^2

P: first+last 15*4
S: middle
F: what two numbers multiply together to get 60 and add together to get 17?

P: 60
S: 17
F: 12,5

Break the middle into two different parts based on the factors:

4x^2 + 12xy + 5xy + 15y^2
4x^2 + 12xy | + 5xy + 15y^2

Factorise each part:

4x(x+3y) + 5y(x+3y)

Bring the outsides together and the common one:

(4x+5y)(x+3y)

 

[Menomaths]

You could also use PSF (product, sum, factors)

4x^2 + 17xy + 15y^2

P: first+last 15*4
S: middle
F: what two numbers multiply together to get 60 and add together to get 17?

P: 60
S: 17
F: 12,5

Break the middle into two different parts based on the factors:

4x^2 + 12xy + 5xy + 15y^2
4x^2 + 12xy | + 5xy + 15y^2

Factorise each part:

4x(x+3y) + 5y(x+3y)

Bring the outsides together and the common one:

(4x+5y)(x+3y)

Can do this in a few seconds with a calculator allowed in HSC

 

[Hypem]

ok, what am i doing wrong here?

I'm asked to rearrange this to make k the subject?

T = 2pi√m/k
T^2 = 4pi^2 m/k <--Divide on this by K?
T^2/k = 4pi^2 m
KT^2 = 4pi^2 m
k = 4pi^2m/T <- I got this ==''

is this right?

ohh you meant sqrt(m/k)
nvm