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Solutions to Combinatorics Problems from before. Taken from AMC competition and Ross 8ed.
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3. TEXAS – 6! CALIFORNIA – 9!/(2!2!) MISSISSIPPI – 11!/(4!4!2!)
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4. a. 6^6 b. (6C2) c. We have 3! Orderings of color from {R,G,B} (6C2) choices for first color (first two sides) (4C2) choices for second color (third and fourth sides) (2C2) choices for third color (fiWh and sixth sides) In total: 3! * (6C2)(4C2) = 720 coloring schemes d.
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We can think about this problem in several ways. Say we have n people in a room # handshakes = nC2 # handshakes = (n-‐1)2/2 The first person has (n-‐1) people to shake hands with, the second (n-‐2), the third (n-‐3), so #handshakes = (n-‐1) + (n-‐2) + (n-‐3) + … + 1
6.Square in The Way: Solugon • Each path of length 8 must either go through the II or IV
quadrant – we can assume symmetry, and only focus on paths through the fourth quadrant.
• We can only ever have a path through the two starred pts Through the red star point there is only 1 possible
path of length 8 A path through the green star point consists of two paths, the path to the star and from the star For the path given: [RRRU][RUUU] We can choose U in 4C1 ways for the first subpath And R in 4C1 ways for the second subpath
In total, #paths = 2 ( 1 + (4C1)2) = 34 paths