Question

Graph the function y = 2x3 – x2 – 4x + 5. To the nearest tenth, over which interval is the function decreasing?

Answer 1

As you progress in math, it will become increasingly important that you know how to express exponentiation properly.

y = 2x3 – x2 – 4x + 5  should be written    y = 2^x3 – x^2 – 4^x + 5.  The 
" ^ " symbol denotes exponentiation.

I see you're apparently in middle school.  Is that so?  If so, are you taking calculus already?  If so, nice!

Case 1:  You do not yet know calculus and have not differentiated or found critical values.  Sketch the function          y = 2x^3 – x^2 – 4^x + 5, including the y-intercept at (0,5).  Can you identify the intervals on which the graph appears to be increasing and those on which it appears to be decreasing?

Case 2:  You do know differentiation, critical values and the first derivative test.  Differentiate  y = 2x^3 – x^2 – 4^x + 5 and set the derivative = to 0:

dy/dx = 6x^2 - 2x - 4 = 0.  Reduce this by dividing all terms by 2:

dy/dx = 3x^2 - x - 2 = 0    I used synthetic div. to determine that one root is x = 2/3.  Try it yourself.  This leaves the coefficients of the other factor, (3x+3); this other factor is x = 3/(-3) = -1.  Again, you should check this.

Now we have 2 roots:  -1 and 2/3

Draw a number line.  Locate the origin (0,0).  Plot the points (-1, 0) and (2/3, 0).  This subdivides the number line into 3 subintervals:

(-infinity, -1), (-1, 2/3) and (2/3, infinity).

Choose a test number from each interval and subst. it for x in the derivative formula above.  If the derivative comes out +, the function is increasing on that interval; if -, the function is decreasing.

Ask all the questions you want, if this explanation is not sufficiently clear.

Answer 2

Answer:

Option 3 - (-0.7,1)  

Step-by-step explanation:

Given : Function  $y=2x^3-x^2-4x+5$

To find : Over which interval is the function decreasing?

Solution :

First we plot the graph using graphing calculator.

Refer the attached figure below.

Now, Examining the graph

From $-\infty$ to (-0.667,6.63) the graph is increasing as the curve is increasing.

From (-0.667,6.63) to (1,2) the graph decreasing as the curve is decreasing.

From  (1,2) to $\infty$ the graph is increasing as the curve is increasing.

So, The interval in which the function is decreasing is given by (-0.667,1)

Round to nearest tenth,  (-0.7,1)

Therefore, Option 3 is correct.