   SEARCH HOME Math Central Quandaries & Queries  Question from Mark: What is y as an expression in terms of x? Hi Mark,

When you commonly see this is where you have an equation involving $x$ and $y$ and perhaps involving other variables and you are asked to write $y$ as an expression in terms of $x.$ This means that you are to manipulate the equation you have to obtain an equivalent equation of the form

$y = \mbox{ an expression containing x but not y.}$

For example suppose the original equation is $x^2 + 3y - 7 - x - y = 0.$ First you only want terms containing $y$ on the left side of the equation so add $-x^2 +7 + x$ to each side to get

$x^2 + 3y - 7 - x - y -x^2 + 7 + x = -x^2 +7 + x$

which simplifies to

$3y - y = x^2 +7 + x \mbox{ or } 2y = x^2 + x + 7.$

Multiplying each side by $\large \frac12$ yields

$y = \frac12 \left(x^2 + x + 7\right)$

which is equivalent to the original equation $x^2 + 3y - 7 - x - y = 0$ but written as $y$ as an expression in terms of $x.$

I hope this helps,
Penny       * Registered trade mark of Imperial Oil Limited. Used under license. Math Central is supported by the University of Regina and the Imperial Oil Foundation.