## Sunday, November 29, 2015

### [AM_20151130IAXS] A Motif for the Absolutely Absolute

Problem

Introduction
This question would pose a challenge for many students, although theoretically it is within reach of a good Additional Mathematics student (~ grade 10).  Graphs of both  sin x  and  cos x  are waves that oscillate up and down.  There are many pairs of vertical bars, indicating the absolute values or modulus, and these seem confusing.

Strategy
Let us graph the functions  y = |cos x|   and  y = |sin x|.   Note that  ||sin x| – |cos x|| = ||cos x| – |sin x||.   The absolute difference of  |cos x|   and  |sin x|  is the difference between them ignoring the negative sign (if any) of the result.  And this is just the difference between the higher value and the lower value.
Can you see any repeating patterns?  [H04]  Can you visualise the required area?  How many times is that of the basic pattern (known as “motif” in art)?  [H09, H10, H11]

Solution

Remarks
Our total area is made up of four congruent pieces.  When  0 < x < p/4,  cos x  is higher than  sin x.  That allows us to strip away all the absolute signs and do the calculation.

H04. Look for pattern(s)
H09. Restate the problem in another way
H10. Simplify the problem
H11. Solve part of the problem

Suitable Levels
challenge for GCE ‘O’ Additional Mathematics  IB Mathematics SL HL
GCE ‘A’ Level H2 Mathematics  IB Mathematics HL
AP Calculus AB & BC
* University / College calculus
* other syllabuses that involve integration and area
* whoever is game for a challenge in integration