This is a fairly common problem for IB. I explain it this way
Elasticity is a measure of change - and the the answer depends on the size
of the change. To be 100% accurate you must differentiate the equation for
the curve - i.e. use a change which is infinitely small. Page 66 of
Sloman1999 is succinct and brilliant on this.
For A level and IB this is not required, so learn the formula for arc
elasticity.
.But "candidates should be aware that there are alternative formulae, such
as the mid-point formula" see page 17 of the syllabus.
In the case of the IB (I think) they say - that where there is a conflict -
like yours then the method to be used is TR method - there was a note in a
circular or mark scheme a few years back saying this - can any of you IB
teachers remember? However, if you have the answer from the IB for that
question you will know their position on it.
Chris Rodda
Website
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