The Redskins confirmed today that they have obtained defensive end Fred Cook from the Baltimore Colts. Although terms were not announced, it is believed Washington gave up a 1982 ninth-round pick and a 1983 sixth-round choice, the latter hinging on how much Cook plays this season.

"For what we gave up, this trade is no gamble at all," said Richie Petitbon, the Redskin defensive coordinator. "We have a potentially good football player who really fell off last season. Now we have to find out whether that was a fluke or whether he can't play any more."

Cook has been in the NFL since 1974, when he was drafted on the second round out of Southern Mississippi by the Colts. He was a standout on what had been a fine defensive front, but slumped to only two sacks and 56 solo tackles last season. He had demanded to be traded or released.

If Cook can play, it is likely he will become a starter at Coy Bacon's defensive end spot, with young players Mat Mendenhall and Dexter Manley as backups for him and Karl Lorch. That would make Bacon the odd man out, although the 37-year-old pass rushing specialist still is not being ruled out as a potential contributor this season.

There also is the possibility that Cook who once had 15 sacks in a season and has 57 career sacks, can't play full time at age 29 after starting 104 consecutive games. So he could be employed as a pass-rushing specialist along with Manley, with Lorch and Mendenhall starting on the early downs.

Whatever happens, it seems certain the Redskins will begin the season with a rebuilt front four, with tackle Dave Butz and Lorch the only holdover regulars and Wilbur Young a new starter at tackle. Last year, the Redskins applied inconsistent pressure on the quarterback and were horrendous trying to stop the run.

"We have nine players (including Pat Ogrin and Perry Brooks) in the line who can help us," Petitbon said. "We just have to see what to do with them." The Redskins probably will keep no more than seven lineman and perhaps as few as six.

This was General Manager Bobby Beathard's ninth trade since April 7. In that span, he has obtained 12 draft choices plus Joe Washington, Young, Henry Childs and Cook.He has given up Childs, Jeff Williams and up to 10 picks, including his 1982 first-round selection. He also signed free agent Terry Metcalf, giving up a choice.

Coupled with the draft, in which they picked 12 players, the Redskins are convinced they have upgraded significantly their three most glaring weaknesses from last year: offensive line, defensive line and running back. There could be eight new starters (out of a possible 11) in those three areas.