Question #110503
Find the interval of unit length that contains the smallest positive root of the
equation f(x) = x^3 −5x^2+1 = 0. Starting with this interval, find an interval of
length 0.05 or less that contains the root, by Bisection method.
1
Expert's answer
2020-04-20T13:12:13-0400

Use Regular False method to determine where the smallest positive root is.

Choose two points a and b such that f(a) and f(b) are of opposite signs.


f(0)=1,f(1)=3.f(0)=1, \\ f(1)=-3.

With a=0 and b=1 find the (first) value of point of approximation:


x=x1=af(b)bf(a)f(b)f(a)=0(3)11(3)1=0.25. f(x1)=0.703.x=x_1=\frac{af(b)-bf(a)}{f(b)-f(a)}=\frac{0\cdot(-3)-1\cdot1}{(-3)-1}=0.25.\\ \space\\ f(x_1)=0.703.

Therefore, the interval of unit length that contains the smallest positive root is [0;1].


Starting with this interval, find an interval of length 0.05 or less that contains the root, by Bisection method

Consider our interval [xL;xR]=[0;1][x_L;x_R]=[0;1]. Find the center of the interval and the value of the function at this point:


xc=xL+xR2=0+12=0.5, f(xc)=0.125.x_c=\frac{x_L+x_R}{2}=\frac{0+1}{2}=0.5,\\ \space\\ f(x_c)=-0.125.



Check at what part of the interval - [xL;xC][x_L;x_C] or [xc;xR][x_c;x_R] - there is a root: if the values of the function at the ends of one of these intervals are opposite, there is a root:

f(0)f(0.5)<0,f(0.5)f(1)>0.f(0)f(0.5)<0,\\f(0.5)f(1)>0.

Hence, we will only consider [0;0.5][0;0.5]. Perform the next iteration by bisection method:


xc=0+0.52=0.25,f(xc)=0.703.Is the root on the left or right of xc?f(0)f(0.25)>0,f(0.25)f(0.5)<0.x_c=\frac{0+0.5}{2}=0.25,\\ f(x_c)=0.703.\\ \text{Is the root on the left or right of } x_c?\\ f(0)f(0.25)>0,\\f(0.25)f(0.5)<0.

Hence, we will only consider [0.25;0.5]. Perform the next iteration:


xc=0.25+0.52=0.375,f(xc)=0.35.Is the root on the left or right of xc?f(0.25)f(0.375)>0,f(0.375)f(0.5)<0.x_c=\frac{0.25+0.5}{2}=0.375,\\ f(x_c)=0.35.\\ \text{Is the root on the left or right of } x_c?\\ f(0.25)f(0.375)>0,\\f(0.375)f(0.5)<0.

Hence, we will only consider [0.375;0.5]. Perform the next iteration:


xc=0.375+0.52=0.438,f(xc)=0.127.Is the root on the left of xc or on the right?f(0.438)f(0.375)>0,f(0.438)f(0.5)<0.x_c=\frac{0.375+0.5}{2}=0.438,\\ f(x_c)=0.127.\\ \text{Is the root on the left of } x_c\text{ or on the right?}\\ f(0.438)f(0.375)>0,\\f(0.438)f(0.5)<0.

Hence, we will only consider [0.438;0.5]. Perform the next iteration:

xc=0.438+0.52=0.469,f(xc)=0.0034.Is the root on the left of xc or on the right?f(0.438)f(0.469)>0,f(0.469)f(0.5)<0.x_c=\frac{0.438+0.5}{2}=0.469,\\ f(x_c)=0.0034.\\ \text{Is the root on the left of } x_c\text{ or on the right?}\\ f(0.438)f(0.469)>0,\\f(0.469)f(0.5)<0.

But the length of both of these intervals is less than 0.05, therefore, the smallest positive root lies between 0.469 and 0.5.


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