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Write an appropriate direct variation equation if y = 30 when x = -10.

I'm going into 12th garde, but I've been playing catch up since 7th. I really need help

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Darlene N. | Experienced Math Teacher and Doctoral Candidate in Math EducationExperienced Math Teacher and Doctoral Ca...
5.0 5.0 (3 lesson ratings) (3)
Direct variation means one variable is a constant multiple of the other.  The formula for direct variation is y = kx, where k is the constant of variation. If you are studying direct variation, this is what your teacher expects to see, as k is also involved in other form of variation that you will study later.
The first step is to solve for k using the information we know. The given point is (-10,30). So...
30 = k * -10
k = -3
Therefore, your equation is now y = -3x.
Philip P. | Effective and Affordable Math TutorEffective and Affordable Math Tutor
5.0 5.0 (443 lesson ratings) (443)
The basic equation for a straight line is:
y = mx + b
Where m is the slope of the line and b is the y-intercept.  The term "direct variation" means that the y-intercept is zero (b = 0); that is, the line crosses through the origin (0,0), so that the equation is:
y = mx
So all we need to do is find the slope.  The formula for the slope is the change in y over the change in x between any two points, (x1,y1) and (x2,y2), on the line:
m = (y2-y1) / (x2-x1)
So to find m we need two points on the line.  The problem gives us one (-10,30).  Because this line is a "direct variation", we have the second point, (0,0).
m = (30-0) / (-10-0) = -30/10 = -3
Hence the equation of the line is:
y = -3x