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Algebra
1.
(11 p.)
Let \( a \) be the coefficient of \( x^2 \) in the polynomial \[ (1x)(1+2x)(13x)\dots (1+14x)(115x).\] Determine \( a \)
2.
(19 p.)
Consider the set \( S\subseteq(0,1]^2 \) in the coordinate plane that consists of all points \( (x,y) \) such that both \( [\log_2(1/x)] \) and \( [\log_5(1/y)] \) are even. The area of \( S \) can be written in the form \( p/q \) for two relatively prime integers \( p \) and \( q \). Evaluate \( p+q \).
3.
(31 p.)
Let \( a_1,a_2,... \) be a sequence defined by \( a_1=1 \) and \[ a_{n+1}=\sqrt {a_n^22a_n+3}+1\] for \( n \ge 1 \). Find \( a_{513} \).
4.
(23 p.)
Let \( f:\mathbb N\rightarrow\mathbb R \) be the function defined by \( f(1) = 1 \), \( f(n) = n/10 \) if \( n \) is a multiple of 10 and \( f(n) = n+1 \) otherwise. For each positive integer \( m \) define the sequence \( x_1 \), \( x_2 \), \( x_3 \), ... by \( x_1 = m \), \( x_{n+1} = f(x_n) \). Let \( g(m) \) be the smallest \( n \) such that \( x_n = 1 \). (Examples: \( g(100) = 3 \), \( g(87) = 7 \).) Denote by \( N \) be the number of positive integers \( m \) such that \( g(m) = 20 \). The number of distinct prime factors of \( N \) is equal to \( 2^u\cdot v \) for two nonnegative integers \( u \) and \( v \) such that \( v \) is odd. Determine \( u+v \).
5.
(14 p.)
Let \( a \), \( b \), \( c \), \( d \) be the roots of \( x^4  x^3  x^2  1 = 0 \). Find \( p(a) + p(b) + p(c) + p(d) \), where \( p(x) = x^6  x^5  x^3  x^2  x \).
20052021
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