JEE Advance - Mathematics (2009 - Paper 2 Offline - No. 1)
Explanation
To solve this problem, we need to find the integer points $$\left( {x,\,y,\,z} \right)$$ that satisfy the given system of homogeneous equations:
$$\matrix{ {3x - y - z = 0} \cr { - 3x + z = 0} \cr { - 3x + 2y + z = 0} \cr }$$
Firstly, let’s solve for $$z$$ in terms of $$x$$ from the second equation:
$$ - 3x + z = 0 \Rightarrow z = 3x $$
Next, substitute $$z = 3x$$ into the first equation:
$$ 3x - y - 3x = 0 \Rightarrow -y = 0 \Rightarrow y = 0 $$
With $$y = 0$$ and $$z = 3x$$, the third equation also should be satisfied. Let's substitute $$y$$ and $$z$$ back into the third equation to verify:
$$ - 3x + 2y + z = 0 \Rightarrow - 3x + 2(0) + 3x = 0 $$
This equation holds true, confirming that the solutions for $$y$$ and $$z$$ remain consistent. Therefore, the points that satisfy the given system are of the form:
$$\left( x,\,0,\,3x \right)$$
Additionally, we need $$x^2 + y^2 + z^2 \le 100$$. Substituting $$y = 0$$ and $$z = 3x$$, we get:
$$ x^2 + 0^2 + (3x)^2 \le 100 $$
This further simplifies to:
$$ x^2 + 9x^2 \le 100 $$
$$ 10x^2 \le 100 $$
$$ x^2 \le 10 $$
Hence, $$ -\sqrt{10} \le x \le \sqrt{10} $$
Since $$x$$ must be an integer, we evaluate acceptable values for $$x$$:
$$x \in \{-3,\,-2,\,-1,\,0,\,1,\,2,\,3\}$$
For each of these values, let’s determine the corresponding points $$\left( x,\,0,\,3x \right)$$:
- $$( -3,\,0,\,-9 )$$
- $$( -2,\,0,\,-6 )$$
- $$( -1,\,0,\,-3 )$$
- $$( 0,\,0,\,0 )$$
- $$( 1,\,0,\,3 )$$
- $$( 2,\,0,\,6 )$$
- $$( 3,\,0,\,9 )$$
Thus, there are a total of 7 such points.
Therefore, the number of integer-coordinate points $$\left( x,\,y,\,z \right)$$ satisfying the given system of equations and the condition $$x^2 + y^2 + z^2 \le 100$$ is 7.
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