geno3141

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 #1
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One way to calculate this: Find the points that make this equation true:  -(7c - 18)(2c) = 0

                                           -(7c - 18)(2c) = 0

Divide both sides by -1:       (7c - 18)(2c) = 0

Either          7c - 18  =  0         or         2c  =  0

                           7c  =  18                     c  =  0

                             c  =  18/7

These two points separate the number line into three sections:

      the section where  c  <  0,  

      the section where  0 <  c  < 18/7,

      the section where  c  >  18/7

Try a number from the section where  c  <  0; try any negative number (I'll pick -2) in the original problem:      -(7c - 18)(2c) > 0     --->     -(7·-2 - 18)(2·-2)  =  -7(-32)(-4) > 0    this doesn't work!

Try a number from the section 0 <  c  < 18/7; I'll pick 1.                                                                                      -(7c - 18)(2c) > 0     --->     -(7·1 - 18)(2·1)  =  -7(-11)(2) > 0    this works!

Try a number from the section where  c  >  18/7; I'll pick 10.                                                                              -(7c - 18)(2c) > 0     --->     -(7·10 - 18)(2·10)  =  -7(52)(20) > 0    this doesn't work!

So the solution is  0 < c < 18/7.

(If there had been a  ≥  or a  ≤  sign in the original problem, there would be this type of sign in the answer.

19 oct 2014