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 #2
avatar+893 
+5

Equations like this need a numerical (iterative) method for their solution.

First though is to sketch a graph or graphs in order, if possible, to determine the number and approximate locations of the roots. For this particular example, best is to rewrite the equation as x2 - 4x = 2x and to sketch the graphs of y = x2 - 4x, which is a concave up parabola crossing the x-axis at x=0 and x=4, and y = 2x which looks rather like the y = ex curve. At their point(s) of intersection x2 - 4x = 2x in which case their x co-ordinates will be the solutions to the original equation. There is just one intersection to the left of the origin, located (after trying two or three values for x), round about x = -0.2, and no intersections to the right, (the 2x curve being already at a height of 16 as the parabola becomes positive at x=4).

The most popular numerical method for solving non-linear equations is probably the Newton-Raphson method.

For this equation the formula is

$$x_{n+1}=x_{n}-\frac{x^{2}_{n}-4x_{n}-2^{x_{n}}}{2x_{n}-4-2^{x_{n}}\ln(2)}$$

which, starting with x0 = -0.2 converges to 0.206100 after just two iterations.

It's also usually possible to obtain a convergent iterative formula from the original equation. Rewriting the original equation as x = (x2 - 2x)/4 and using this as an iterative formula, (substitute x=-0.2 on the rhs to get x=-0.2076, substitute this into the rhs etc.), produces the sequence -0.2, -0.2076, -0.2057, -0.2062, -0.20608, etc. Not as fast as Newton-Raphson but it will get there eventually.

May 4, 2014
 #119
avatar+118725 
+9

@@ End of Day Wrap - Sunday 4/5/14   Sydney, Australia Time 19:20

Hi everyone,

Our wonderfully impressive answers were posted by CPhill, Problem, Bertie, Alan, Kitty

I had real problems getting into this thread.  I kept being told "File not found" I guess there is still quite a few ants at the picnic!  

Another recurring complaint is that time stamps on out messages jump all over the "clock" 

Andre has now installed a fraction calculator.  There is a drop down menu in the top right corner of the calculator.  There is a few different calculators available here.

web2 drop down box

My favourite post today was written by a guest calling himself "Reluctantly Modern Old Geezer"  It was VERY funny!     http://web2.0calc.com/questions/why-does-my-oldish-texas-instruments-not-have-a-prb-button

There were also some output discussion in a couple of different posts.  The allowable matrix inputs are 'sloppy' and the equaton solution output puts an extra equal sign in the middle which, I think, is universally disliked by mathematicians.  Here are the threads

http://web2.0calc.com/questions/if-3-4x-1-what-is-x_1#r103588

http://web2.0calc.com/questions/4x-5--3x-4-3x-10-1#r103592

For those of you interested in higher level maths there were these posts.

http://web2.0calc.com/questions/1-i-16-1-i-15   (complex numbers)

http://web2.0calc.com/questions/taylor-series-for-1-cos-x-2-around-a-0 (Taylor series)

I think that about covers it.

Enjoy the rest of your weekend.

Melody

melodymathforum@gmail.com

May 4, 2014

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