# Solution to puzzle 77: Minimal straight cut

A piece of wooden board in the shape of an isosceles right triangle, with sides 1, 1, , is to be sawn into two pieces.  Find the length and location of the shortest straight cut which divides the board into two parts of equal area.

We consider two cases:

• Cut 1, across one of the acute angles.
• Cut 2, across the right angle.

## Cut 1

Let X lie on AB with AX = x, and Y lie on AC with AY = y.  Then XY is a straight cut of length z.

Area ABC = ½ × base × perpendicular height = ½.

Area AXY, considering AX as the base, equals ½ × x × (y/) = xy / (2).

Since Area AXY = ½ × Area ABC, we have xy = 1/.

Applying the law of cosines (also known as the cosine rule) to AXY:

 z2 = x2 + y2 − 2xy cos A = x2 + y2 − 1,  since cos A = 1/ = (x − y)2 + ( − 1)

Hence the minimum value of z2 (and therefore of z) occurs when x = y, so that z2 = − 1.
Then, since xy = 1/, x = y = 1/.

Intuitively, it seems clear that cutting across the smaller angle, as above, will yield a shorter minimal cut than cutting across the right angle.  We verify this intuition below.

## Cut 2

Let Y lie on AB with BY = y, and X lie on BC with BX = x.  Then XY is a straight cut of length z.

Area BXY = ½xy.

Since Area BXY = ½ × Area ABC, we have xy = ½.

Applying Pythagoras' Theorem to BXY:

 z2 = x2 + y2 = (x − y)2 + 1

Hence the minimum value of z2 occurs when x = y, so that z2 = 1.
This is longer than the minimal length established for cut 1, above.

## Minimal cut

Therefore the minimal straight cut has length , with, in the first diagram, AX = AY =    (Of course, by symmetry, there is an equivalent cut of equal length from BC to AC.)

## Remarks

It is natural to ask whether a shorter cut is possible if we are not restricted to using a straight line.  The answer is: yes!  To see why, we use symmetry.

Consider the diagram below, obtained by successive reflection of the triangle in its sides.  The area of the whole square is 4; the area of the (regular) octagon is 2.

A result known as the isoperimetric theorem states that of all planar shapes with the same area the circle has the shortest perimeter.  Hence a circle with the same area as the octagon will have minimal perimeter.  It then follows that the minimal arc which bisects an isosceles right triangle, while passing through one leg and the hypotenuse, is an arc of such a circle, with its center at a 45° vertex of the triangle.  See below.

(In order to prove that this is the shortest arc that bisects an isosceles right triangle, we need to show that no other arc, such as one passing through both legs, is shorter.  This can be confirmed by a similar argument based upon symmetry.)

It is easy to show that the shortest arc, shown above, has length 0.626657, versus 0.643594 for the straight line bisector.