The value of y varies directly with the value of x, and y = –21 when x = 3.

Which equations represent the relationship between y and x?

Choose exactly two answers that are correct.

Question

The value of y varies directly with the value of x, and y = –21 when x = 3.
 
Which equations represent the relationship between y and x?
 
Choose exactly two answers that are correct.

igreen · Answer

Answers?

anonymous · Answer

-7x=y 

 y/x=-7

  x/y=7  

xy=-7

igreen · Answer

y = kx

This is the formula for direct variation.

Plug in y = -21 and x = 3:

-21 = k(3)

-21 = 3k

Divide 3 to both sides, what's -21 / 3?

anonymous · Answer

-7?

igreen · Answer

Yes, so -7 is our constant, plug that back in the equation:

y = kx

y = -7x

This is one of our equations.

igreen · Answer

Now divide 'x' to both sides.

anonymous · Answer

-2.3?

igreen · Answer

No, it doesn't equal anything, we're just moving 'x' to the other side by dividing.

anonymous · Answer

Oh... Well up above you said x=3. Sorry D:

igreen · Answer

$y = -7x$

$\dfrac{y}{x} = \dfrac{-7x}{x}$

The x's on the right cancel out:

$\dfrac{y}{x} = \dfrac{-7 \cancel x}{\cancel x}$

Leaving us with:

$\dfrac{y}{x} = -7$

igreen · Answer

Those are our 2 equations :)

igreen · Answer

$y = -7x$

and

$\dfrac{y}{x} = -7$

anonymous · Answer

Thank you! :D