Sometimes we might be given an equation that contains more than one variable and asked to isolate one of the variables. This means that we need to put the variable we're interested in on one side of the equation (on its own) and everything else on the other.

For example, suppose we want to isolate n in the equation 2n - k = q + 5k.

The equation has three variables, n, k, and q. However, we will treat the variables k and q as constants because we are solving for n only. We can use the addition and multiplication principles to solve for n\mathbin{.}

First, we add k to both sides:

\eqalign{ 2n - k &= q + 5k \\[5pt] 2n - k + k &= q + 5k +k \\[5pt] 2n &= q + 6k \\ }

Then, we divide both sides of the equation by 2\mathbin{:}

\eqalign{ 2n &= q + 6k \\[5pt] \dfrac {2n} {2} &= \dfrac {q+6k} {2} \\ n &= \dfrac {q+6k} {2} }

Because the equation is now solved for n , we say that we've made n the subject of the equation.

FLAG

Solve for x where 2x - y = 5y - z.

EXPLANATION

We need to isolate x, so we will treat the other variables y and z as constants. We can use the addition and multiplication principles to solve for x.

First, we add y to both sides:

\eqalign{ 2x - y &= 5y - z \\[5pt] 2x - y + y &= 5y - z + y \\[5pt] 2x &= 6y - z }

Then, we divide both sides by 2\mathbin{:}

\eqalign{ 2x &= 6y - z \\[5pt] \dfrac {2x} 2 &= \dfrac{6y - z} {2} \\[5pt] x &= \dfrac{6y - z} {2} }

FLAG

If $2w +3x = y,$ then $w=$

a
b
c
d
e

Make $c$ the subject of the equation $-6a=9b-2c.$

a
$c=9b+3a$
b
$c=\dfrac{9b+6a}{2}$
c
$c=\dfrac{9b-6a}{2}$
d
$c=9b+6a+2$
e
$c=9b+6a-2$

The formula for the perimeter of a rectangle is given by $$ p = 2w + 2l, $$

where $w$ is the width of the rectangle and $l$ is the length.

What is the formula for the length of a rectangle in terms of its perimeter and width?

a
b
c
d
e

The energy E of a body at rest is related to its mass m by the equation E=mC , where C is a constant. What is an expression for the mass ( m ) in terms of the energy ( E ) and the constant ( C )?

EXPLANATION

We need to isolate m, so we will treat the other variables E and C as constants. Since m is being multiplied by C, we need to divide both sides of the equation by C : \require{cancel} \begin{align} E &=mC\\[5pt] \dfrac E C &=\dfrac{m\cancel{C}}{\cancel{C}}\\[5pt] \dfrac E C &=m \end{align} Thus, m = \dfrac E C.

Note: This result assumes that C\neq 0.

FLAG

If $a = bc,$ then $c=$

a
b
c
d
e

If $w = 4xyz,$ then $z=$

a
b
c
d
e

The average velocity of a body in free fall is given by $$ v = \dfrac{1}{2}gt, $$

where $g$ is gravity and $t$ is the time that the body has fallen.

What is the formula for the time in terms of the other variables?

a
$t= 2v - g$
b
$t= \dfrac{g}{2v}$
c
$t= 2vg$
d
$t= \dfrac{2v}{g}$
e
$t= 2v + g$

The total energy E of a body moving with a (squared) velocity V can be approximated using the equation E = mC + \dfrac{1}{2}mV. Find an expression for V in terms of the other quantities.

EXPLANATION

We need to isolate V, so we will treat the other variables E, C, and m as constants. We can use the addition and multiplication principles to solve for V.

First, we subtract mC from both sides of the equation: \begin{align*} E &= mC + \dfrac{1}{2}mV\\[5pt] E - mC &= mC + \dfrac{1}{2}mV - mC \\[5pt] E - mC &=\dfrac 1 2 mV \end{align*}

Then, we multiply by 2 to get rid of the fraction:

\begin{align*} E - mC &=\dfrac 1 2 mV\\[5pt] 2\cdot (E-mC) &=2\cdot \dfrac 1 2 mV\\[5pt] 2(E-mC) &=mV\\[5pt] \end{align*}

Finally, we divide by m to get V alone:

\begin{align*} 2 (E-mC) &=mV\\[5pt] \dfrac{2 (E-mC)}{m} &=\dfrac{mV}{m}\\[5pt] \dfrac{2(E-mC)}{m} &=V \end{align*}

Thus, V = \dfrac{2(E-mC)}{m}.

Note: This result assumes that m\neq 0.

FLAG

If $5cd = a+b,$ then $c = $

a
b
c
d
e

If $\dfrac{pq}{4} - 2r = 5p,$ then $q = $

a
b
c
d
e

A company calculates each employee's monthly salary using the formula $$ s = b+rh, $$

where $b$ is the base salary (which is fixed regardless of the number of overtime hours worked), $r$ is the hourly rate for overtime work, and $h$ is the number of overtime hours worked.

What is the formula for the number of overtime hours worked in terms of the other variables?

a
b
c
d
e
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