foo.c: In function 'main':
foo.c:9:20: warning: 'sum' is used uninitialized [-Wuninitialized]
    9 |                 sum=c+sum;
      |                 ~~~^~~~~~
foo.c:6:25: note: 'sum' was declared here
    6 |    double a=1.0,b=1.0,c,sum,x;
      |                         ^~~

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