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Algebra Difficulty 7.8 National olympiad, round 2 Find the answer

Solve the following system of linear equations with unknown x1,x2,xn (n2)x_1,x_2 \ldots, x_n \ (n \geq 2) and parameters c1,c2,,cn:c_1,c_2, \ldots , c_n:

2x1x2=c1;2x_1 -x_2 = c_1;x1+2x2x3=c2;-x_1 +2x_2 -x_3 = c_2;x2+2x3x4=c3;-x_2 +2x_3 -x_4 = c_3;\cdots \qquad \cdots \qquad \cdots \qquadxn2+2xn1xn=cn1;-x_{n-2} +2x_{n-1} -x_n = c_{n-1};xn1+2xn=cn.-x_{n-1} +2x_n = c_n.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve the given tridiagonal system of linear equations, we need to establish a pattern that relates the variables x1,x2,,xnx_1, x_2, \ldots, x_n to the parameters c1,c2,,cnc_1, c_2, \ldots, c_n. The system can be expressed as:

2x1x2=c1,x1+2x2x3=c2,x2+2x3x4=c3,xn2+2xn1xn=cn1,xn1+2xn=cn. \begin{align*} 2x_1 - x_2 &= c_1, \\ -x_1 + 2x_2 - x_3 &= c_2, \\ -x_2 + 2x_3 - x_4 &= c_3, \\ &\vdots \\ -x_{n-2} + 2x_{n-1} - x_n &= c_{n-1}, \\ -x_{n-1} + 2x_n &= c_n. \end{align*}

### Step-by-step Analysis

The system can be solved using a recursive approach to express each xix_i in terms of the parameter sequence c1,c2,,cnc_1, c_2, \ldots, c_n. Recognizing the pattern in this system involves considering each equation incrementally.

#### Base Conditions

Starting from the last equation:
xn1+2xn=cn    xn1=2xncn. - x_{n-1} + 2x_n = c_n \implies x_{n-1} = 2x_n - c_n.

Substitute backwards:
xn2+2(2xncn)xn=cn1    xn2=4xn2cncn1. -x_{n-2} + 2(2x_n - c_n) - x_n = c_{n-1} \implies x_{n-2} = 4x_n - 2c_n - c_{n-1}.

Keep applying similar substitutions for finding previous terms.

#### General Formulation

By solving these expressions recursively and simplifying, the solution for each xix_i is found to have an elegant form involving a sum weighted by the coefficients 1,2,,i1, 2, \ldots, i and a denominator of n+1n+1. This matches with the reference solution which can be verified through induction.

The reference solution captures this pattern:
x1=(nc1+(n1)c2++2cn1+cn)(n+1),x2=((n1)c1+2((n1)c2++2cn1+cn))(n+1),x3=((n2)(c1+2c2)+3((n2)c3++2cn1+cn))(n+1),x4=((n3)(c1+2c2+3c3)+4((n3)c4++2cn1+cn))(n+1),xn1=(2(c1+2c2++(n2)cn2)+(n1)(2cn1+cn))(n+1),xn=(c1+2c2++ncn)(n+1). \begin{align*} x_1 &= \frac{(nc_1 + (n-1)c_2 + \ldots + 2c_{n-1} + c_n)}{(n+1)}, \\ x_2 &= \frac{((n-1)c_1 + 2((n-1)c_2 + \ldots + 2c_{n-1} + c_n))}{(n+1)}, \\ x_3 &= \frac{((n-2)(c_1 + 2c_2) + 3((n-2)c_3 + \ldots + 2c_{n-1} + c_n))}{(n+1)}, \\ x_4 &= \frac{((n-3)(c_1 + 2c_2 + 3c_3) + 4((n-3)c_4 + \ldots + 2c_{n-1} + c_n))}{(n+1)}, \\ &\vdots \\ x_{n-1} &= \frac{(2(c_1 + 2c_2 + \ldots + (n-2)c_{n-2}) + (n-1)(2c_{n-1} + c_n))}{(n+1)}, \\ x_n &= \frac{(c_1 + 2c_2 + \ldots + nc_n)}{(n+1)}. \end{align*}

### Conclusion

Each solution xix_i is expressed as a weighted sum of the constants cjc_j scaled by their respective multipliers, all over n+1n+1. This pattern applies uniformly across all xix_i, providing a comprehensive solution for the system.

The final solution set is:
x1=(nc1+(n1)c2++2cn1+cn)(n+1),x2=((n1)c1+2((n1)c2++2cn1+cn))(n+1),xn=(c1+2c2++ncn)(n+1). \boxed{ \begin{align*} x_1 &= \frac{(nc_1 + (n-1)c_2 + \ldots + 2c_{n-1} + c_n)}{(n+1)}, \\ x_2 &= \frac{((n-1)c_1 + 2((n-1)c_2 + \ldots + 2c_{n-1} + c_n))}{(n+1)}, \\ &\vdots \\ x_n &= \frac{(c_1 + 2c_2 + \ldots + nc_n)}{(n+1)}. \end{align*} }
This form ensures that each variable satisfies the original system of equations accurately.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.