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AQA A-level Maths Change of signs method

Change of signs method

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Suppose we have 2 points a and b, and some function f(x). Provided f(x) is a continuous function, if f(a) < 0 and f(b) > 0, then we know f(x) crosses the x–axis between a and b, therefore having a root in that interval. This is increadibly obvious if one looks at a graph of f(x).

Graphical representation of the change of signs method

Consider the graph shown with f(a) < 1 and f(b) > 1. Any continuous line f(x) between a and b must cross the point f(x) = 1. The change of sign method can be used to locate solutions for any equation, provided it can be rearranged to the form f(x) = 0. Also, using the change of sign method can help solve an equation. One can just strategically guess roots and narrow down the interval we know the root is in. In general, for more complicated functions, the change of sign method can be really useful to find where a root is, without analytically solving the equation for f(x) = 0.

Change of signs method

There's some issues with this method one should be aware of. Firstly, there are some occasions a function has a root without a change of signs. Consider the function shown, which never has a change of sign, but still has a root. A change of sign means there is a root – but having a root does not mean there has to be a change of sign. If your interval is big enough, the function may have a couple of roots between 2 points where f(x) has the same sign. Also, the change of sign method only works with continuous functions – there can't be any jumps or gaps.

Limits of the change of sign method

The change of signs method can also be used in more problem solving contexts. It can be useful for any continuous function which could, for example, give a velocity value. Perhaps one would use the change of signs method on an equation which was the result of equating 2 equations together (for the intersection of them both).

Change of signs method problems in context

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