## Roots of an Equation - Changes of Sign

Suppose we are to solve an equation\[f(x)=0\]

. We may not be able to solve the equation exactly, but we may be able to say that the solution lies between two numbers \[x_1, \: x_2\]

if \[f(x)\]

changes sign between these values of \[x\]

. We may be able to narrow the interval by finding smaller intervals on which there is a sign change.Suppose

\[x^2-2x-1=0\]

,When

\[x=2, : x^2-2x-1=2^2-2 \times 2-1=-1 \lt 0\]

When

\[x=3, : x^2-2x-1=3^2-2 \times 3-1=2 \gt 0\]

There is a sign change between

\[x=2, \: x=3\]

so \[x^2-2x-1=0\]

for some \[2 \lt x \lt 3\]

.We can narrow this interval.

When

\[x=2.5, \: x^2-2x-1=2.5^2-2 \times 2.5-1=0.25 \gt 0\]

Hence the sign change is for some

\[2.5 \lt x \lt 3\]

.