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0_0_39296464_4829.cpp:13:13: error: 'sqrt' was not declared in this scope
   13 | double d1 = sqrt(2);    //根号二
      |             ^~~~
0_0_39296464_4829.cpp: In function 'int main()':
0_0_39296464_4829.cpp:24:52: error: 'sqrt' was not declared in this scope
   24 |                 dp[i] = dp[i - 1] + (i - 1) * d1 + sqrt(i * i + (i - 1) * (i - 1));
      |                                                    ^~~~