Find a root of an equation `f(x)=2x^3-2x-5` using False Position method (regula falsi method)
Solution:
Here `2x^3-2x-5=0`
Let `f(x) = 2x^3-2x-5`
Here
`1^(st)` iteration :
Here `f(1) = -5 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1` and `x_1 = 2`
`x_2 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_2 = 1 - (-5) * (2 - 1)/(7 - (-5))`
`x_2 = 1.41667`
`f(x_2)=f(1.41667)=2*1.41667^(3)-2*1.41667-5=-2.14699 < 0`
`2^(nd)` iteration :
Here `f(1.41667) = -2.14699 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.41667` and `x_1 = 2`
`x_3 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_3 = 1.41667 - (-2.14699) * (2 - 1.41667)/(7 - (-2.14699))`
`x_3 = 1.55359`
`f(x_3)=f(1.55359)=2*1.55359^(3)-2*1.55359-5=-0.60759 < 0`
`3^(rd)` iteration :
Here `f(1.55359) = -0.60759 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.55359` and `x_1 = 2`
`x_4 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_4 = 1.55359 - (-0.60759) * (2 - 1.55359)/(7 - (-0.60759))`
`x_4 = 1.58924`
`f(x_4)=f(1.58924)=2*1.58924^(3)-2*1.58924-5=-0.15063 < 0`
`4^(th)` iteration :
Here `f(1.58924) = -0.15063 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.58924` and `x_1 = 2`
`x_5 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_5 = 1.58924 - (-0.15063) * (2 - 1.58924)/(7 - (-0.15063))`
`x_5 = 1.59789`
`f(x_5)=f(1.59789)=2*1.59789^(3)-2*1.59789-5=-0.0361 < 0`
`5^(th)` iteration :
Here `f(1.59789) = -0.0361 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.59789` and `x_1 = 2`
`x_6 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_6 = 1.59789 - (-0.0361) * (2 - 1.59789)/(7 - (-0.0361))`
`x_6 = 1.59996`
`f(x_6)=f(1.59996)=2*1.59996^(3)-2*1.59996-5=-0.00858 < 0`
`6^(th)` iteration :
Here `f(1.59996) = -0.00858 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.59996` and `x_1 = 2`
`x_7 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_7 = 1.59996 - (-0.00858) * (2 - 1.59996)/(7 - (-0.00858))`
`x_7 = 1.60045`
`f(x_7)=f(1.60045)=2*1.60045^(3)-2*1.60045-5=-0.00203 < 0`
`7^(th)` iteration :
Here `f(1.60045) = -0.00203 < 0` and `f(2) = 7 > 0`
`:.` Now, Root lies between `x_0 = 1.60045` and `x_1 = 2`
`x_8 = x_0 - f(x_0) * (x_1 - x_0)/(f(x_1) - f(x_0))`
`x_8 = 1.60045 - (-0.00203) * (2 - 1.60045)/(7 - (-0.00203))`
`x_8 = 1.60056`
`f(x_8)=f(1.60056)=2*1.60056^(3)-2*1.60056-5=-0.00048 < 0`
Approximate root of the equation `2x^3-2x-5=0` using False Position method is `1.60056`
`n` | `x_0` | `f(x_0)` | `x_1` | `f(x_1)` | `x_2` | `f(x_2)` | Update |
1 | 1 | -5 | 2 | 7 | 1.41667 | -2.14699 | `x_0 = x_2` |
2 | 1.41667 | -2.14699 | 2 | 7 | 1.55359 | -0.60759 | `x_0 = x_2` |
3 | 1.55359 | -0.60759 | 2 | 7 | 1.58924 | -0.15063 | `x_0 = x_2` |
4 | 1.58924 | -0.15063 | 2 | 7 | 1.59789 | -0.0361 | `x_0 = x_2` |
5 | 1.59789 | -0.0361 | 2 | 7 | 1.59996 | -0.00858 | `x_0 = x_2` |
6 | 1.59996 | -0.00858 | 2 | 7 | 1.60045 | -0.00203 | `x_0 = x_2` |
7 | 1.60045 | -0.00203 | 2 | 7 | 1.60056 | -0.00048 | `x_0 = x_2` |
This material is intended as a summary. Use your textbook for detail explanation.
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