Math Is Fun Forum

bilat

Member

Registered: 2008-04-23

Posts: 4

Questions

Starting with x = 3 use trial and improvement to find correct to 2 decimals places the value of x which satisfies:
                             2X x X x X + 3X = 112

By the way X x X x X means X to the third power

Thank you if you answer.

I don't no how to do it.

Bila

cushydom

Member

Registered: 2008-04-23

Posts: 10

Re: Questions

put three into (cuberoot(112-3*x))/2. then put this answer back in. and so on.
I am not sure if this counts as trial and improvement

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Questions

You need to use x=3 in the left-hand side of that equation and see what comes out.
If it's lower than 112, then 3 is too low a value for x; it's it's higher then 3 is too high.

2x3³ + 3x3 = 56 + 9 = 65, so x is higher than 3.
Now try x=4, for example. 2x4³ + 3x4 = 128 + 12 = 140, so x is lower than 4.

We now know that x is between 3 and 4, and with each trial the range of possible values gets reduced. With enough trials, you'll be able to tell what x is, accurate to 2 decimal places.

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Why did the vector cross the road?
It wanted to be normal.