Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
The figure shows the graph of a function on the closed interval . It is made of two straight pieces and one arc of a parabola, joined end to end:
So , , and , and there are no gaps in the graph. Two new functions are now built from on the open interval . The first is : it keeps the parts of the graph above the -axis and reflects the parts below it. The second is : it keeps the right half of the graph of and copies it onto the left half, as a mirror image in the -axis. Work from the formulas, and use the figure to see what happens near each special point.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) On each piece is a polynomial, so the only candidates are and . At the slopes are (left) and (right). At they are (left) and (right). Both are corners: 2 points.
(b) keeps the two corners of , since and are not zero, and is or near them. New trouble can come only where : at and (on the straight pieces) and at (on the arc). At and the graph crosses the axis with slopes and . At each of them has slope just to the left and just to the right: two new corners. At the arc only touches the axis: , so near , , a smooth parabola with slope 0. So fails at , , and : 4 points.
(c) For , , and for , . So has the corner of at and its mirror image at . At : near 0, , with slopes on the left and on the right, another corner. The point of itself plays no part, because uses only on . So fails at , and : 3 points.
(d) At , , so near , and . So .
(e) means or . On : gives , and gives . On : gives , and gives . On the arc runs from just above up to and back down to , so it never reaches again ( only at and ). The solutions are : 4. The one at is easy to miss, because only touches the level 1 there and does not cross it.
Check. Sampling the slopes of , and every along and looking for sudden jumps finds exactly these corners, and none at .
Watch out. Counting in (b) is the usual slip. A zero of gives a corner of only when changes sign there with a nonzero slope.
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