Question

if y varies directly as x and y=12 when x=8, find y when x= 14.
Please help me with this. i have no idea how to do it

Answer 1

y varies directly with x
y=kx
12=k*8

therefore
k=12/8=3/2


NOw when x=14 (substitute the value of k)
y=kx
y=(3/2)*14
y= .. ?

Answer 2

Did you understand ?

Answer 3

so u divide 3 and 2 and then times that by 14? i have like 8 problems that deal with this and ive done them all wrong....do u mind helping me with them?

Answer 4

Sure ,but you've to remember this

When it varies directly we have to use
y=kx or x=ky (according to the question )

when it varies inversely
x=k*(1/y) or y=k(1/x) (according to the question)

Answer 5

so then what the anwser would be is 14?

Answer 6

No
we got
y= 3/2 * 14

Answer 7

so 21

Answer 8

Right!

Answer 9

:)

Answer 10

Will you help me with the others? please and thank you

Answer 11

Sure :) Post it

Answer 12

suppose y varies jointly as x and z. Find y when x=9 and z=-3, if y=-50 when z is 5 and x is -10

Answer 13

Ok so according to the question

y=(k)xz
Now you need to find the constant k

Answer 14

@RiyaMoon
put the values of y , x & z and find k

Answer 15

-44? i really understand this at all....

Answer 16

In the question its given y varies jointly as x and z

that means y is directy proportional

so we take the equation as y=(k)xz

do you understand this ?

Answer 17

Not really... weres K?

Answer 18

K is a constant of variation.

Answer 19

YOu have to remember this

when y varies jointly/directly with x
y=kx


when x varies jointly/directly with y
x=ky

got it ?

Answer 20

some what yea

Answer 21

" y=-50 when z is 5 and x is -10"
here y ,x & z are given
so you can easily find out k using the equation y=(k)xz

and use the value of k to find out y when x=9 and z=-3

Answer 22

-50=(k) 9 x -3?

Answer 23

Nope
read the question again

"Find y when x=9 and z=-3"
So you have to use this x & z to find out y in the end

now we have to find out k using y=-50 z= 5 and x = -10

Answer 24

so then 5/-10? this is confusing me even more...

Answer 25

y=(k)xz

got this ?

Answer 26

yes but it doesnt help that theres so many numbers...

Answer 27

Its very easy dont worry :)

Answer 28

you should be careful while choosing the numbers

Answer 29

Lol it might be easy for someone who likes math, math has always been my down fall

Answer 30

"Find y when x=9 and z=-3'

Now you know the equation y=(k)xz

so can you find y with this ?

No because you have two unknown variables y & k

Answer 31

So we move on to the next part of the question , where all the values are given

" if y=-50 when z is 5 and x is -10"

Now can you use this to find out k ? (Yes ,using the equation y= (k) xz

Answer 32

How do i plug them in? i just did this and u said i did it wrong

Answer 33

y=(k)xz

-50=(k)5*(-10)

Answer 34

so you do 5 * (-10)? first ?

Answer 35

Yes

Answer 36

-50

Answer 37

Right :)
Now what's k ?

Answer 38

-50?

Answer 39

Nope you're doing it wrong

y=(k)xz
-50= (k)*5(-10)
-50=k*(-50)

Now what's k?

Answer 40

-50k?

Answer 41

Nope

-50=k(-50)

Divide both sides by -50

now what do you get?

Answer 42

k=1

Answer 43

Right :D

Answer 44

Now can you find y when x=9 and z=-3

Answer 45

so 1= 9/-3?

Answer 46

where is y ?

Answer 47

y isnt found yet. so would it be 9/-3*1?

Answer 48

Nope
why do you have to divide 9/-3 ?

remember the equation
y=(k)xz

Answer 49

urg this is so confusing.....

Answer 50

so then itss 1*9*-3?

Answer 51

Yes Exactly :)

Answer 52

So what is y?

Answer 53

-27

Answer 54

Right ! :)

Answer 55

kk thanks

Answer 56

Anytime =)