Questions   
Sort: 
 #1
avatar
0

Here is a "ridiculous" solution from Mathematica 11, IF anybody cares to go through it. There surely MUST be a shorter solution!!.

 

Solve for x:
(3 log(x - 2))/(log(10)) = (log(2 x))/(log(10)) - 3

Subtract (log(2 x))/(log(10)) - 3 from both sides:
3 + (3 log(x - 2))/(log(10)) - (log(2 x))/(log(10)) = 0

Bring 3 + (3 log(x - 2))/(log(10)) - (log(2 x))/(log(10)) together using the common denominator log(10):
(3 log(10) + 3 log(x - 2) - log(2 x))/(log(10)) = 0

Multiply both sides by log(10):
3 log(10) + 3 log(x - 2) - log(2 x) = 0

3 log(10) + 3 log(x - 2) - log(2 x) = log(1000) + log((x - 2)^3) + log(1/(2 x)) = log((500 (x - 2)^3)/x):
log((500 (x - 2)^3)/x) = 0

Cancel logarithms by taking exp of both sides:
(500 (x - 2)^3)/x = 1

Multiply both sides by x:
500 (x - 2)^3 = x

Subtract x from both sides:
500 (x - 2)^3 - x = 0

Expand out terms of the left hand side:
500 x^3 - 3000 x^2 + 5999 x - 4000 = 0

Eliminate the quadratic term by substituting y = x - 2:
-4000 + 5999 (y + 2) - 3000 (y + 2)^2 + 500 (y + 2)^3 = 0

Expand out terms of the left hand side:
500 y^3 - y - 2 = 0

Divide both sides by 500:
y^3 - y/500 - 1/250 = 0

Change coordinates by substituting y = z + λ/z, where λ is a constant value that will be determined later:
-1/250 + 1/500 (-z - λ/z) + (z + λ/z)^3 = 0

Multiply both sides by z^3 and collect in terms of z:
z^6 + z^4 (3 λ - 1/500) - (z^3)/(250) + z^2 (3 λ^2 - (λ)/(500)) + λ^3 = 0

Substitute λ = 1/1500 and then u = z^3, yielding a quadratic equation in the variable u:
u^2 - u/250 + 1/3375000000 = 0

Find the positive solution to the quadratic equation:
u = (450 + sqrt(202485))/225000

Substitute back for u = z^3:
z^3 = (450 + sqrt(202485))/225000

Taking cube roots gives (450 + sqrt(202485))^(1/3)/(10 15^(2/3)) times the third roots of unity:
z = (450 + sqrt(202485))^(1/3)/(10 15^(2/3)) or z = -(-450 - sqrt(202485))^(1/3)/(10 15^(2/3)) or z = ((-1)^(2/3) (450 + sqrt(202485))^(1/3))/(10 15^(2/3))

Substitute each value of z into y = z + 1/(1500 z):
y = (sqrt(202485) + 450)^(1/3)/(10 15^(2/3)) + 1/(10 (15 (sqrt(202485) + 450))^(1/3)) or y = (-1)^(2/3)/(10 (15 (sqrt(202485) + 450))^(1/3)) - (-sqrt(202485) - 450)^(1/3)/(10 15^(2/3)) or y = ((-1)^(2/3) (sqrt(202485) + 450)^(1/3))/(10 15^(2/3)) - 1/10 ((-1)/(15 (sqrt(202485) + 450)))^(1/3)

Bring each solution to a common denominator and simplify:
y = ((sqrt(202485) + 450)^(2/3) + 15^(1/3))/(10 15^(2/3) (450 + sqrt(202485))^(1/3)) or y = ((-15)^(2/3) - (-15)^(1/3) (sqrt(202485) + 450)^(2/3))/(150 (450 + sqrt(202485))^(1/3)) or y = -((sqrt(202485) - 450)^(1/3) - (-1)^(2/3) (sqrt(202485) + 450)^(1/3))/(10 15^(2/3))

Substitute back for x = y + 2:
x = (15^(1/3) + (sqrt(202485) + 450)^(2/3))/(10 15^(2/3) (sqrt(202485) + 450)^(1/3)) + 2 or x = ((-15)^(2/3) - (-15)^(1/3) (sqrt(202485) + 450)^(2/3))/(150 (sqrt(202485) + 450)^(1/3)) + 2 or x = 2 - ((sqrt(202485) - 450)^(1/3) - (-1)^(2/3) (450 + sqrt(202485))^(1/3))/(10 15^(2/3))

(3 log(x - 2))/(log(10)) ≈ -2.41495 + 2.759 i
(log(2 x))/(log(10)) - 3 ≈ -2.41495 + 0.030247 i:
So this solution is incorrect

(3 log(x - 2))/(log(10)) ≈ -2.36393
(log(2 x))/(log(10)) - 3 ≈ -2.36393:
So this solution is correct

(3 log(x - 2))/(log(10)) ≈ -2.41495 - 2.759 i
(log(2 x))/(log(10)) - 3 ≈ -2.41495 - 0.030247 i:
So this solution is incorrect

The solution is:
Answer: | x = (15^(1/3) + (sqrt(202485) + 450)^(2/3))/(10 15^(2/3) (sqrt(202485) + 450)^(1/3)) + 2 = 2.1629388880535156

Jun 4, 2017
Jun 3, 2017
 #1
avatar+501 
+1
Jun 3, 2017
 #2
avatar+501 
+1
Jun 3, 2017
 #2
avatar+501 
+2
Jun 3, 2017
 #4
avatar+2446 
+2

You can also solve this system algebraically, too, but it requires some logical thinking. You solve it similarly as that of a system of equations. Here are your inequalities:

 

1. \(2x+4y\leq12\)

2. \(3x-y<2\)

 

I'll use substitution. I'll solve for on both equations. I'll solve for on the inequality \(2x+4y\leq12\):

 

\(2x+4y\leq12\) Here is the original inequality. Subtract 2x first to begin isolating y.
\(4y\leq-2x+12\) Divide by 4 on both sides of the equation
\(y\leq-\frac{1}{2}x+3\) Finally, is isolated
   

 

Next, isolate in the inequality 2, \(3x-y<2\):

\(3x-y<2\) This is the original inequality. Subtract 3x on both sides
\(-y<-3x+2\) Divide by -1 on both sides. Remember that the inequality sign flips when dividing by a negative number!
\(y>3x-2\)  
   

 

Our inequalities have changed to from their original to being solved for y:

  1. \(y\leq-\frac{1}{2}x+3\)
  2. \(y>3x-2 \)

This is where the logical thinking comes to play.If 3x-2 is less than y and \(-\frac{1}{2}x+3\) is equal to and greater than y, then \(3x-2<-\frac{1}{2}+3\)! Now that there is one variable in this inequality, we can solve for x.

 

 

\(3x-2<-\frac{1}{2}x+3\) This is what we concluded above. Add 2 on both sides
\(3x<-\frac{1}{2}x+5\) Add (1/2)x on both sides
\(\frac{7}{2}x<5\) Multiply by 2/7 to isolate x.
\(x<\frac{10}{7}<1\frac{3}{7}\)  
   

 

Yet again, it requires a bit of logic again. Plug in 10/7 into both inequalities! Yes, you must plug it into both inequalities! You'll see why after the calculations are made. First, I'll plug into the first and second equation

 

\(y\leq\bf{-\frac{1}{2}*\frac{10}{7}}+3\) Simplify the right hand side by evaluating \(-\frac{1}{2}*\frac{10}{7}\)first.
\(y\leq-\frac{5}{7}+3\) Change 3 to an improper fraction so you can add the fractions together!
\(y\leq-\frac{5}{7}+\frac{21}{7}\) Add the fractions now because of the common denominators!
\(y\leq\frac{16}{7}\)  
   


Great! Let's plug in for the second equation and see what we get:

 

\(y>3(\frac{10}{7})-2\) Multiply 3 by 10/7 first
\(y>\frac{30}{7}-2\) Convert the 2 to an improper fraction
\(y>\frac{30}{7}-\frac{14}{7}\) Subtract the fractions to get the solution set for the other equation!
\(y>\frac{16}{7}\)  
   

 

Now that we know our x- and y-values, let's write them out as a solution set:

 

\(((x<\frac{10}{7},\frac{16}{7}

 

I guess you could stop here and move on to the next problem, but do you notice how y is really equal to every number? Currently, the solution set is saying that y is greater than 16/7, equal to 16/7 and less than 16/7. That's a verbose way of saying that y can be any of the real numbers. I'll write it out for you. Sorry, I had a hard time making the real-number symbol in LaTeX:
 

\((x\in{\rm I\!R}| x<\frac{16}{7},y\in{\rm I\!R})\)

 

You are done now because you have identified the possible values for x and y! You can verify this solution set by graphing. Luckily, Cphill has already done that! You can check it out, if you'd like.

 

One last note before you go!

 

I spent around 3 hours in total (with a slight respite in between) to generate this response. I truly attempted to provide a complete answer with comprehensible explanations. I hope it helped! Also, if anyone sees a typo, please tell me. I'm 95% confident that I missed one or two.


1 Online Users