• Jul 3rd 2005, 11:20 AM
Hayabusa
Suppose y varies directly as x and y=27 when x=3. Find x when y = 45.

I need help with one.
• Jul 3rd 2005, 11:43 AM
MathGuru
Direct variation
Suppose y varies directly as x and y=27 when x=3. Find x when y = 45.

'Varies directly' most likely means an equation of the form y = x + k
Where k is a constant. So as x increases by some amount, y increases by that same amount.

change in y = change in x

y2 - y1 = x2 - x1
45 - 27 = x2 - 3
18 = x2 - 3
21 = x2

when y = 45, x = 21
• Jul 3rd 2005, 11:47 AM
Hayabusa
Thank you so much I just forgot the formula then. :)
• Jul 3rd 2005, 12:41 PM
ticbol
Here is one way.

"y varies directly as x"
y ------> x
y = kx ----the equation.

y=27 when x=3
27 = k*3
k = 27/3 = 9

When y = 45,
45 = 9x
x = 45/9 = 5 ---answer.

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Another meaning of "y varies directly as x" is:
if x goes higher, then y goes higher too; if x goes lower, then y goes lower too....proportionately.
• Jul 3rd 2005, 12:44 PM
MathGuru
Varies directly
Ticbol's answer may be more correct than mine.

It depends on what is meant by varies directly.
• Jul 3rd 2005, 02:20 PM
Hayabusa
thanks again :D