Maths Olympiad Prep

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Geometry Difficulty 6.5 National olympiad Find the answer

Given a quadratic function yy with respect to xx. yy takes the values of 1,2-1, -2 and 11 for the values of x=1, 0, 1x=-1,\ 0,\ 1 respectively.
Find the length of the line segment of the graph of the curve cut by the xx-axis.

1956 Tokyo Institute of Technology entrance exam

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

Solution

1. Determine the quadratic function:
Given the values of y y for specific x x values, we can set up a system of equations to find the coefficients of the quadratic function y=ax2+bx+c y = ax^2 + bx + c .

- For x=1 x = -1 , y=1 y = -1 :
a(1)2+b(1)+c=1    ab+c=1(Equation 1) a(-1)^2 + b(-1) + c = -1 \implies a - b + c = -1 \quad \text{(Equation 1)}
- For x=0 x = 0 , y=2 y = -2 :
a(0)2+b(0)+c=2    c=2(Equation 2) a(0)^2 + b(0) + c = -2 \implies c = -2 \quad \text{(Equation 2)}
- For x=1 x = 1 , y=1 y = 1 :
a(1)2+b(1)+c=1    a+b+c=1(Equation 3) a(1)^2 + b(1) + c = 1 \implies a + b + c = 1 \quad \text{(Equation 3)}

2. Solve the system of equations:
Substitute c=2 c = -2 from Equation 2 into Equations 1 and 3:
- Equation 1:
ab2=1    ab=1(Equation 4) a - b - 2 = -1 \implies a - b = 1 \quad \text{(Equation 4)}
- Equation 3:
a+b2=1    a+b=3(Equation 5) a + b - 2 = 1 \implies a + b = 3 \quad \text{(Equation 5)}

Add Equations 4 and 5 to eliminate b b :
(ab)+(a+b)=1+3    2a=4    a=2 (a - b) + (a + b) = 1 + 3 \implies 2a = 4 \implies a = 2

Substitute a=2 a = 2 back into Equation 4:
2b=1    b=1 2 - b = 1 \implies b = 1

Therefore, the quadratic function is:
y=2x2+x2 y = 2x^2 + x - 2

3. Find the roots of the quadratic function:
To find where the curve cuts the x x -axis, set y=0 y = 0 :
2x2+x2=0 2x^2 + x - 2 = 0

Use the quadratic formula x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} :
a=2,b=1,c=2 a = 2, \quad b = 1, \quad c = -2
x=1±1242(2)22=1±1+164=1±174 x = \frac{-1 \pm \sqrt{1^2 - 4 \cdot 2 \cdot (-2)}}{2 \cdot 2} = \frac{-1 \pm \sqrt{1 + 16}}{4} = \frac{-1 \pm \sqrt{17}}{4}

4. Calculate the distance between the roots:
The roots are x1=1+174 x_1 = \frac{-1 + \sqrt{17}}{4} and x2=1174 x_2 = \frac{-1 - \sqrt{17}}{4} .

The distance between the roots is:
1+1741174=17+174=2174=172 \left| \frac{-1 + \sqrt{17}}{4} - \frac{-1 - \sqrt{17}}{4} \right| = \left| \frac{\sqrt{17} + \sqrt{17}}{4} \right| = \left| \frac{2\sqrt{17}}{4} \right| = \frac{\sqrt{17}}{2}

The final answer is 172\boxed{\frac{\sqrt{17}}{2}}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.