How to use Linux command(sed?) to delete specific lines in a file?

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http://stackoverflow.com – Now I have a file. In it , I have a matrix. For example, I have : 1 a 2 b 2 b 5 b 3 d 4 b 4 b 7 b I know it's easy to use sed command to delete specific lines with specific strings. But what if I only want to delete those lines where the second field's value is b(i.e., second line and fourth line)? (HowTos)