Why The AMC’s Are Trivial

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Why the AMC’s are TrivialBrandon JiangJanuary 24, 20161How to Use this DocumentThis could possibly be used as a sort of study guide, but its main intent is to offer students some direction to prepare for this contest other than just doing pastproblems. Note that it is assumed that the reader is mathematically capableof understanding the standard curriculum at school. If not, the author recommends a different hobby such as biology or debate. Painstakingly crafted, thiswill make all (most) future problems encountered by the reader trivial. Enjoy!2AlgebraIt should be noted that most problems in this contest are algebra based, meaningthat most solutions will require some use of algebra. Besides the topics below,one should have a solid grasp on basic algebraic manipulation and solving equations. In addition, the author would like to comment that contest algebra is thesource of the most contrived problems on the test.2.12.1.1ManipulationIdentitiesOne should definitely be aware of the following identities: a2 b2 (a b)(a b) (difference of squares) a2 2ab b2 (a b)2 and a2 2ab b2 (a b)2 (squares of sums) (a b c)2 a2 b2 c2 2(ab bc ca) (square of sum of 3 variables) a3 b3 (a b)(a2 ab b2 ) and a3 b3 (a b)(a2 ab b2 ) (differenceof cubes) a3 b3 c3 3abc (a b c)(a2 b2 c2 ab bc ca)If he or she wishes, the curious student may ask the author on factoring techniques for contrived multivariable polynomials such as the last one in the list.1

2.1.2SubstitutionsThere is little that can be taught about these, except that if you chose the rightone the result is typically very powerful. The author’s best advice is to chooseone that best fits the problem; for example if you have many factors of x 5 inan expression consider making the substitution y x 5. Linear substitutions are generally useful in this manner; it should be notedthat substituting in order to arrive at symmetric expressions tends to beuseful as well. A certain substitution that problem writers enjoy is what the author hasaptly named the Vieta substitution: Generally in two variable expressionswith, say, a and b one may find the substitution u a b and v abvery useful, it should be noted that with this definition we also have thata and b are the roots of the quadratic x2 ux v.2.2FunctionsKnow the basic definitions of functions; most notably a function may not havetwo different outputs for a single input. (Vertical line test)2.2.1Polynomials Make sure you at least know what a polynomial is. There is bound to bea couple questions related to polynomials on the exam. Be familiar with the Binomial Theorem, which states that n nn n 1n n 2 2nn n(a b)n a ab ab · · · abn 1 b .012n 1nAlso be able to recognize coefficients of a polynomial Pascal’s triangle soyou can manipulate to make a perfect nth power polynomial. Know Vieta’s relations regarding symmetric sums of the roots of a polynomial. For an nth degree polynomial P (x) an xn an 1 xn 1 · · · a1 x a0 with real coefficients, roots r1 , r2 , · · · , rn and an 6 0, an 1 r1 r2 · · · rn ,anan 2 r1 r2 r2 r3 · · · rn r1 ,an···a0( 1)n r1 r2 · · · rn .anThe first and last (sum and product, respectively) are by far the mostuseful.2

Be able to factor polynomials that are intended to be factored. (Generally this means you can easily guess its roots using the Rational RootTheorem). Know that r is a root of P (x) if and only if P (r) 0; a corollary of thisis that the remainder when P (x) is divided by x a is P (a). It may be helpful to write a polynomial in it’s factored form, orP (x) an (x r1 )(x r2 ) · · · (x rn ). Know that P (1) gives the sum of the coefficients of P (x). Be aware of complex roots and basic operations with complex numbers,and consider learning both polar and exponential form. Consider learning about the roots of unity, or the roots of xn 1. Notethat 1 is always a factor and roots of unity should come to mind whensomeone sees an expression like 1 x x2 · · · xn since xn 1 (x 1)(1 x · · · xn 1 ).2.2.2Other functions Be familiar with the six trigonometric functions and their properties, mostnotably which ones are even/odd, Pythagorean identities, and sum anddifference identities. Know the basic properties of exponents and logarithms (sum, product,change-of-base, etc.) A certain function that comes up is the floor or ceiling function, be aware oftheir definition. Note that it is a common technique to write bxc x {x}where {x} denotes the fractional part of x. (It is between 0 and 1). Makeuse of the fact that bxc x and that the floor function always returns aninteger. The absolute value function also may appear, just be aware of how itworks and know the generalized triangle inequality, which states that forcomplex numbers ai ,nnXX ai ai .i 1i 1 For piece-wise defined functions, be sure to observe function behavior onborders. Sometimes, there will be a problem with very sophisticated looking notation but in reality is not too difficult. Be able to overcome notation onsome of these weird functions.3

Be on the lookout for cyclic functions. If you see a rational function, thereis a chance it will be cyclic, thus making things a lot easier. (This meansthat for some positive integer n, f n (x) f (x)).2.3Sequences Know that arithmetic sequences have a common difference and geometricsequences have a common ratio. Know formulas for the sums of sequences (series). When asked to evaluate a complicated sum or product, list a few termsto see if cancelation of terms occurs. Be aware of telescoping sequences.Most prominent ismX1n(n 1)n 1mX11 nn 1n 11mNote that all the middle terms cancel, leaving us with 1 m 1 m 1.Sometimes factoring then decomposing an expression like this will allowyou to collapse the entire thing like above. Be creative! Know the following series:n(n 1)2nn 1– 1 2 3 ··· n 2– 1 2 2 ··· 2 2– 1 3 5 · · · 2n 1 n– 12 22 32 · · · n2 – 13 23 33 · · · n3 3(triangular numbers) 12n(n 1)(2n 1)6 2n(n 1)2GeometryFor some reason, both the author and contest writers love geometry, even thoughnearly all Euclidian geometry is trivial with coordinates (contingent on computational prowess and time). Geometry tends to be forgotten among typicalstudents due to the fact that the public school curriculum only offers one yearof basic geometry. If you wish to do well on this contest, it is imperative thatyou get decent at geometry. The author stresses the importance of drawing adiagram when solving most geometry problems.4

3.1Similarity If two objects are similar, then their respective length ratios are equivalentand all angles are the same. The most common way to show this is withtwo equal angles. The technique of finding angles in a problem is called angle chasing, whichis probably the most useful skill in contest geometry. Angle chasing generally comes down to alternate interior angles, parallellines, and angles in a triangle summing to 180 . Sometimes similar triangles are not given, and you will have to make themyourselves. The most common way (accompanying angle chasing) to dothis is dropping perpendiculars or by drawing parallel lines. Continuing from above, always make note of right angles, since they usually make things a lot easier.3.2CirclesA circle is the locus of points equidistant from its center. These figures are quiteprominent in geometry, and are extremely useful for angle chasing. Commontechniques are to draw radii to tangents, draw common tangents, and draw thesegment between the centers of the circles. Area of a circle: πr2 . Circumference of a circle: 2πr.3.2.1Useful Angle-Chasing Facts: Central angles, with the vertex at the center, have their measure equal tothe measure of the arc. Inscribed angles equal to half the measure of the arc, and as a resultinscribed angles subtending to the same arc are equal. Be aware of thecase when one ray is tangent to the circle. As a result, inscribed angles with two points diametrically opposite eachother have a measure of 90 . The angle formed between two secants when the vertex lies outside thecircle is half the difference of the arcs subtended. The angle formed between two chords when the vertex lies within thecircle is half the sum of the two arcs subtended.5

3.2.2Power of a PointThese formulas are almost guaranteed in a circle problem, and provide one withvaluable information on lengths. Let the point P be the intersection of two chords of the circle, say ABand CD. Then,(AP )(P B) (CP )(P D). Let P be the intersection of two secants in a circle, say AB and CD, andlet P E be the tangent from P to the circle at E. Then, we have that(P A)(P B) (P C)(P D) (P E)2 . These formulas generalize to describe the ”power” of a point respect to acircle as a function. This is useful because we know the product of anytwo of those lengths once we know the power. Between two circles, the radical axis is the line on which all points havethe same power respect to both circles. For three circles, the radical axesconcur at a single point called the radical center.3.3TrianglesHopefully you know what a triangle is. Most contest problems rely on this mostbasic polygon of three sides. Sadly, this document will not provide enough information for a certain degree of proficiency regarding triangles; nothing canreplace doing practice problems. Below are very useful results regarding triangles with respect to the arbitrary triangle 4ABC with angles A, B and C,and side lengths (opposite the respective angle) a, b, and c: The angles in a triangle sum to 180 . Lines passing through any vertex of a triangle are called cevians. It istrivial to show that they will intersect exactly one side of the triangle(you may have to extend sides). A median is a cevian intersecting the opposite side at its midpoint. If allthree medians are drawn, they concur at the centroid of the triangle. It iswell known that medians divide each other in a 2 : 1 ratio. An angle bisector is a cevian which bisects the angle at the vertex it passesthrough. The three angle bisectors concur at the incenter of the triangle(the circle within the triangle that is tangent to all three side lengths.(Note: The author is aware that each vertex has two angle bisectors, andtaking that third case gives the result of the three excenters). An altitude is a cevian which is perpendicular to the side opposite thevertex. This is especially useful with all the right angles, and the threealtitudes concur at the orthocenter.6

A useful tool for proving concurrency is Ceva’s Theorem, which states thatfor points D, E, and F on sides BC, CA, and AB, lines AD, BE, and CFare concurrent if and only ifDB EC F A·· 1.DC EA F BTo answer the question of the inquisitive reader, this theorem can proveall the above results (but it is not the only method). The perpendicular bisectors of each side in a triangle concur at the circumcenter (try proving this), or the center of the circle that passes throughpoints A, B and C. Here is a slew of triangle formulas, ranked in order of importance (by theauthor):– Area of a triangle: [ABC] 12 BC · hA 12 CA · hB 21 AB · hC [ABC] 12 bc sin A 12 ca sin B 21 ab sin Cp [ABC] s(s a)(s b)(s c), where s [ABC] rs, where r is the inradius. [ABC] abc4R , where R is the circumradius.a b c2– The Pythagorean Theorem states that for a right triangle with legsa and b and hypotenuse c,a2 b2 c2 .This formula probably gets the most abuse by problem writers. Makesure you also know most primitive Pythagorean triples.– The Triangle Inequality states that for any nondegenerate triangle,a b c, b c a, c a b– The Angle Bisector Theorem states that if the angle bisector of Ameets BC at D, thenABBD .ACCD– The Ratio Lemma is a generalization of the angle bisector theoremand states that for any D on BC,BDAB sin BAD· .AC sin CADCD– The Law of Cosines is a generalization of the Pythagorean theoremand states thatc2 a2 b2 2ab cos C.7

– The Extended Law of Sines states that:abb 2Rsin Asin Bsin C– Stewart’s Theorem states that if D is a point on BC such that AD d, BD m, and CD n, thenman dad bmb cnc.(Or, as the author likes to say, a man and his dad put the bomb inthe sink.)3.4(Mostly Cyclic) Quadrilaterals and Other PolygonsKnow the generic quadrilaterals and their properties (parallelogram, trapezoid,kite, rhombus, etc.) Typically, nothing extremely dependent on definition comesup on the exam, so that should be the least of your worries.3.4.1Cyclic QuadrilateralsA cyclic quadrilateral is one that can be inscribed in a circle. As one wouldexpect, this has some powerful implications. For a cyclic quadrilateral ABCD, ABC CDA BCD DAB 180 . ABD ACD, BCA BDA, CDB CAB and DAC DBC. All of this follows directly from inscribed angles.p The area of a cyclic quadrilateral is s(s a)(s b)(s c)(s d), wheres a b c d.2 Be aware that you can apply power of a point on lengths relating to acyclic quadrilateral. Ptolemy’s theorem states that for a cyclic quadrilateral,AB · CD BC · DA AC · BD.3.4.2Other Quadrilaterals and PolygonsBe able to use properties of other quadrilaterals to solve problems as well. Mostcommon are the trapezoid and the parallelogram. Area of trapezoid:12 (b1 b2 )h. Area of parallelogram: bh. Square and rectangles are special cases of parallelograms; be aware ofcertain angle and side conditions that result.8

Area of a rhombus:12 d1 d2 ,where d1 and d2 are the lengths of its diagonals. For problems involving quadrilaterals, also be on the lookout for similartriangles as well. The sum of the angles in an n-gon is 180(n 2). The number of diagonals in an n-gon isn(n 3).2 Polygon problems generally break down into triangles and quadrilaterals,so