Question 1202252: If y varies directly as x and y=12 when x=-3, find y when x=7
Found 2 solutions by mananth, greenestamps:
Answer by mananth(16946)   (Show Source):
You can put this solution on YOUR website! If y varies directly as x and y=12 when x=-3, find y when x=7
y varies as x
y=kx
y=12 when x=-3
12=k*-3
k=12/-3 =-4
you know k=-4 and x=7
plug in equation to get y
Answer by greenestamps(13200)  (Show Source):
You can put this solution on YOUR website!
If you have a requirement to find the formula relating x and y, for repeated us later on, then you can do as the other tutor did to find the proportionality constant to find the answer.
But if, as your post suggests, you only need to find the answer for this one case, then there is no need to go to all that work.
Since the variation is direct, when the value of x changes the value of y changes by the same factor.
In this example, the value of x changes from -3 to +7, a factor of (-7/3), so the value of y changes by a factor of (-7/3).
ANSWER: y = 12*(-7/3) = 4(-7) = -28
Note that it is very unusual in the real world to have a direct variation in which the values of the variables can be both positive and negative....
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