Order of Operations and Solving Linear EquationsDate: 03/01/2006 at 01:09:23 From: Kevin Subject: How come PEMDAS does not work for solving two step equations How do I explain to 6th graders that when solving x/-2 + 16 = 32 they need to subtract 16 first, versus multiplying by -2, in order to solve for x? Students are taught to work through the proper order of operations following PEMDAS, but the first step in solving this equation should be subtraction, not multiplication. Thanks! Date: 03/01/2006 at 09:47:42 From: Doctor Peterson Subject: Re: How come PEMDAS does not work for solving two step equations Hi, Kevin. Actually, the order of operations is precisely why you want to subtract the 16 first. When you are EVALUATING an expression, you do the multiplication first, then the addition. Here, however, you are not evaluating, but SOLVING, which really means undoing the evaluation. If I knew what x is, I would be first dividing it by -2, and then adding 16. The result is 32. But we want to discover what that secret number is, so we have to work backwards. First we undo the LAST step, the addition, to see what number I added 16 to in order to end up with 32. That number is 32 - 16 = 16. Then, we undo the FIRST step, the division, to see what number I divided by -2 to get 16; we multiply by -2 and find that x is -32. So solving requires following the order of operations backward! I often illustrate this by describing how I put on my shoes and socks. In the morning, I put on my socks first and then my shoes, following the order of operations for dressing. At night, I don't take off my socks first! I first undo the last thing I did, taking off the shoes, and then the socks. Evaluating is dressing a variable, solving is undressing it! If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/ |
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