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Find all values of c such that c^3 + 4c > 5c^2. Answer with interval notation. 

Please enter your response in interval notation. Refer to Formatting Tips below for detailed instructions on formatting your response.

 Oct 26, 2014

Best Answer 

 #2
avatar+130511 
+5

c^3 + 4c > 5c^2      subtract 5c^2 from both sides

c^3 + 4c - 5c^2 > 0   rearrange

c^3 - 5c^2 + 4c > 0     set this = 0 and factor

c^3 - 5c^2 + 4c = 0

c(c^2 -5c + 4) = 0

c(c-4)(c-1) = 0

And setting each factor to 0 , we have that c =0, c = 4 and c =1

Now, adding back the inequality sign, we have

c(c-4)(c-1) > 0   

And if c < 0 ,   c(c-4)(c-1) < 0

And if c > 0 but < 1 ,  c(c-4)(c-1) is > 0

And if c > 1 but < 4,  c(c-4)(c-1) is < 0

And if c > 4, c(c-4)(c-1) > 0

Here's a graph using 'x" rather than "c".........https://www.desmos.com/calculator/pw1vqn74aq

So, interval notation we have

[0 < c < 1]  U   [4 < x < ∞]

 

 Oct 26, 2014
 #1
avatar
0

Cphill please help me on this.

 Oct 26, 2014
 #2
avatar+130511 
+5
Best Answer

c^3 + 4c > 5c^2      subtract 5c^2 from both sides

c^3 + 4c - 5c^2 > 0   rearrange

c^3 - 5c^2 + 4c > 0     set this = 0 and factor

c^3 - 5c^2 + 4c = 0

c(c^2 -5c + 4) = 0

c(c-4)(c-1) = 0

And setting each factor to 0 , we have that c =0, c = 4 and c =1

Now, adding back the inequality sign, we have

c(c-4)(c-1) > 0   

And if c < 0 ,   c(c-4)(c-1) < 0

And if c > 0 but < 1 ,  c(c-4)(c-1) is > 0

And if c > 1 but < 4,  c(c-4)(c-1) is < 0

And if c > 4, c(c-4)(c-1) > 0

Here's a graph using 'x" rather than "c".........https://www.desmos.com/calculator/pw1vqn74aq

So, interval notation we have

[0 < c < 1]  U   [4 < x < ∞]

 

CPhill Oct 26, 2014

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