Maths Olympiad Prep

Library / /10 of 23

Geometry Difficulty 5.0 AIME Find the answer United States

A graph is symmetric about a line if the graph remains unchanged after reflection in that line. For how many quadruples of integers (a,b,c,d)(a, b, c, d), where a,b,c,d5|a|, |b|, |c|, |d| \le 5 and cc and dd are not both 00, is the graph of
y=ax+bcx+d y = \frac{ax + b}{cx + d}
symmetric about the line y=xy = x?

This was a multiple-choice question, but the options didn't survive into the source we have. The answer given is B, and the solution below works it through.

Solution

Answer (B): There are two cases, depending on whether the denominator of the fraction is constant.

CASE 1. Suppose that c=0c = 0. Then the graph of y=ax+bdy = \frac{ax+b}{d} is a line. It is symmetric with respect to the line y=xy = x if the slope of the line is 11 and the line passes through the origin, or the slope is 1-1. A line with slope 11 that passes through the origin occurs if a=d0a = d \neq 0 and b=0b = 0. There are 1010 possible values of aa. The slope is 1-1 if d=a0d = -a \neq 0. There are 1010 possible values of aa, and 1111 possible values of bb, which gives 110110 possibilities. This gives a total of 120120 possibilities in Case 1.

CASE 2. Suppose that c0c \neq 0. The quantity y=ax+bcx+dy = \frac{ax+b}{cx+d} tends to ac\frac{a}{c} as x±x \to \pm\infty. This is a horizontal asymptote. The degree of the polynomial in the numerator can be reduced by subtracting this asymptotic value:
yac=ax+bcx+dac=acx+bca(cx+d)c(cx+d)=bcadc(cx+d)=Δcx+d y - \frac{a}{c} = \frac{ax + b}{cx + d} - \frac{a}{c} = \frac{acx + bc - a(cx + d)}{c(cx + d)} = \frac{bc - ad}{c(cx + d)} = \frac{\Delta}{cx + d}
where
Δ=bcad. \Delta = bc - ad.
If Δ=0\Delta = 0, then the graph of yy consists of the points (x,ac)(x, \frac{a}{c}), except that the point (dc,ac)(-\frac{d}{c}, \frac{a}{c}) is missing because the formula reads 10\frac{1}{0} when x=dcx = -\frac{d}{c}. In any case, this set of points is not symmetric with respect to the line y=xy = x. From now on, assume that Δ0\Delta \neq 0. Multiply both sides by cx+dcx + d to find that
(cx+d)(yac)=bcad. (cx + d)(y - \frac{a}{c}) = bc - ad.
Note that cx+d0cx + d \neq 0 because Δ0\Delta \neq 0. Thus, if it were desired, both sides of the new equation could be divided by cx+dcx + d to recover the former equation. Hence the set of pairs (x,y)(x, y) that satisfy the former equation is exactly the same as the set of (x,y)(x, y) that satisfy the new equation, which determines a rectangular hyperbola. This hyperbola is symmetric with respect to the line y=xy = x if and only if its center (ac,dc)(\frac{a}{c}, -\frac{d}{c}) lies on this line of symmetry. That is, d=ad = -a. Thus Δ0\Delta \neq 0 if and only if bc+a20bc + a^2 \neq 0.

If a=0a = 0, then bb and cc are free to take on any nonzero values, which gives 1010=10010 \cdot 10 = 100 quadruples (0,b,c,0)(0, b, c, 0). If a0a \neq 0, then because d=ad = -a and c0c \neq 0, there are 101110=110010 \cdot 11 \cdot 10 = 1100 quadruples (a,b,c,a)(a, b, c, -a) to consider. Note that bc+a2=0bc + a^2 = 0 if and only if either a=b=c|a| = |b| = |c| and bb and cc have opposite signs, which is 102=2010 \cdot 2 = 20 cases, or a=2|a| = 2 and (b,c)=(±4,1)(b, c) = (\pm 4, \mp 1) or (±1,4)(\pm 1, \mp 4), which is 222=82 \cdot 2 \cdot 2 = 8 additional cases. Thus the total number of possibilities in Case 2 is 100+1100(20+8)=1172100+1100-(20+8) = 1172.

In all there are 120+1172=1292120 + 1172 = 1292 quadruples satisfying the given conditions.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.