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'Ab Surd Ity' printed from http://nrich.maths.org/
Why do this problem?
The problem provides good practice in the manipuation of surds and
in algebra (involving the expansions of $(p+q)^2$ and $(p+q)^3$,
the difference of two squares and the use of the Remainder Theorem
to factorise a cubic equation). If care is taken with the algebra
the result comes out in a satisfyingly neat way. The question looks
complicated but it turns out to be simple.
Although this is a longer, two part, question, the Hint gives
sufficient guidance for this to be set to a class to work on
If 'extra' solutions are introduced by squaring or cubing, how
do you decide which are the correct solutions?
This is how the
problem was used by Peter Thomas, a Sixth Form College
I was absent at a meeting and set the class work to
consolidate topics taught the previous lesson. The work was routine
exercises from a textbook (Emanuel and Wood) which I encouraged
them to approach selectively (what I called 'bread and butter' with
some specific questions as a 'doggy bag' for homework).
Alongside this I set them the four nrich problems as 'cake'
with the instruction to tackle at least one.
Ab Surd Ity (this problem)
Then over the following couple of weeks the problems were
discussed in lessons as they related to the topics being
The reaction was positive. The teacher covering the lesson
reported strong engagement. From the individuals he mentioned this
extended to those who liked to finish quickly then sit back. When
the problems were discussed subsequently the contributions made
suggested most had had a go and got somewhere with at least one
It livened up what could have been a rather boring lesson of