Missing number is 6. All opposite numbers add up to 21.
缺少的数字是6，每组相对数字的和为21。 。
Both Numbers may slide synchronously in opposite directions.
两数可以同步地向相反方向移位。
When the direction is judged the feedback - sector Numbers must be identified. When the minus feedback is formed, the factors in the opposite direction should be odd number.
在化工控制回路中判断调节器作用方向时，可根据回路中负反馈环节个数来确定，当形成回路负反馈时，反作用方向的环节数应为单数个。
When the direction is judged the feedback - sector Numbers must be identified. When the minus feedback is formed, the factors in the opposite direction should be odd number.
在化工控制回路中判断调节器作用方向时，可根据回路中负反馈环节个数来确定，当形成回路负反馈时，反作用方向的环节数应为单数个。
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