Math CentralQuandaries & Queries


Question from Rosie, a student:

Solve equation for y


Hi Rosie,

Often for this type of problem the instructions will read "Solve for y in terms of x." You are not going to obtain a numeric value for y but rather an expression for y which involves the variable x. I do a similar problem.

Solve equation for y

8 + 5x = 9 - 4y

I want an equation with y on the left side and the right side containing x terms and constant terms but no y terms. My first step is to add 4y to each side to obtain

8 + 5x + 4y = 9 - 4y + 4y
8 + 5x + 4y = 9

Next subtract 8 for each side

8 + 5x + 4y - 8 = 9 - 8
5x + 4y = 1

To remove the x term from the left side subtract 5x from each side

5x + 4y - 5x = 1 - 5x
4y = 1 - 5x

All that remains is to divide each side by 4

4y/4 = (1 - 5x)/4
y = 1/45/4 x

Now try your problem,

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