*So the equation will look like this: (c 3) (2c 3) = 33 Now that we have an equation, we can solve for the unknown variable, c.*

In other words, let c represent Charlie's age today.

Since Brian is twice as old as Charlie today, then let 2c represent Brian's age today.

Let's try θ = 30°: sin(−30°) = −0.5 and −sin(30°) = −0.5 So it is true for θ = 30° Let's try θ = 90°: sin(−90°) = −1 and −sin(90°) = −1 So it is also true for θ = 90° Is it true for all values of θ?

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In this problem, we don't know Jack's or Diane's age.

Since Jack's age is expressed in relation to Diane's age (in this problem, Jack is two years older than Diane), so then our variable will be based on Diane's age. If Jack is two years older than Diane, then Jack's age must be: d 2.

We must substitute into Brian's equation to determine his current age. Therefore, Brian must be 18 years old (2c = 2 x 9).

To check our answer, we must substitute into the original equation to verify that the left side of the equation equals the right side.

You can solve these types of problems in four steps.

First, we can express what we don't know as a variable, an alphabetical representation of what we're trying to solve for.

## Comments Problem Solving Using Equations