For the following problem, y varies directly as x. If y = −12 when x = 9, find y when x is 15?

I don't understand how to do these kind of problems. I don't know how to do it.

1 Answer
Jim G.
Apr 29, 2018

#y=-20#

Explanation:

#"the initial statement is "ypropx#

#"to convert to an equation multiply by k the constant"#
#"of variation"#

#rArry=kx#

#"to find k use the given condition"#

#y=-12" when "x=9#

#y=kxrArrk=y/x=(-12)/9=-4/3#

#"the equation is " color(red)(bar(ul(|color(white)(2/2)color(black)(y=-4/3x=-(4x)/3)color(white)(2/2)|)))#

#"when "x=15" then substituting in equation"#

#y=-(4xx15)/3=-60/3=-20#

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