foo.c: In function 'main':
foo.c:10:18: warning: iteration 19 invokes undefined behavior [-Waggressive-loop-optimizations]
   10 |         {b[i+1]=b[i]+b[i-1];
      |                 ~^~~
foo.c:9:18: note: within this loop
    9 |         for(i=2;i<=21;i++)
      |                 ~^~~~
foo.c:12:13: warning: 'sum' is used uninitialized [-Wuninitialized]
   12 |          sum+=(double)a[i-1]/(double)b[i];}
      |          ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:6:16: note: 'sum' was declared here
    6 |         double sum;
      |                ^~~

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