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How to Change the Subject of a Formula

First of all let’s make sure you know what the subject of an equation is. The simplest way to think of it is to say that the subject is a pro-numeral all by itself standing before the equal sign. So in this equation A equals LB, the subject is the ‘A’. Do you recognize this equation? It’s actually the formula for the area of a rectangle. Area equals length times breadth.

Now let’s say you knew the area and the length of a rectangle, and you needed to find out the breadth. Maybe you’re making a vegetable garden and you know how much room the vegetable you want to grow would need, that’s the area. And you know the length of your garden as well. That will be how long the edge of your backyard is. Now, before you pick up that shovel, you’d like to know how far out you need to dig the ground up, so you don’t waste any energy. That is you want to know the breadth of your garden.

So let’s get that B onto the left side by itself. All you have to do is use the rules for opposite operations that you’ve already learned. LB, really means L times B. So do the opposite operation and divide both sides by L. The L’s will cancel on the right leaving just B. And on the left you have A over L. Now just turn around and you’ve got a new subject. B equals A over L. It’s that simple. B is now the subject of the equation.

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