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Hello world!
Hello world!
Hello world!

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#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int main(void)
{
    int a,b;
    scanf("%d%d",&a,&b);
    printf("%d\n",a+b);
    return 0;
}

$$
\frac{\partial u}{\partial t} = h^2 \left( \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2} + \frac{\partial^2 u}{\partial z^2}\right)
$$

电解水：
$$
\require{mhchem}
\ce{2H2O \xlongequal[]{通电} 2H2 ^ + O2 ^}
$$

氢气燃烧：
$$
\require{mhchem}
\ce{2H2 + O2 \xlongequal[]{\Delta} 2H2O}
$$