(k+1)(k-3)>0
２つのものをかけて正だから、両方とも正、または両方とも負の二通あります。
両方とも正のとき、k+1>0かつk-3>0➡k>-1かつk>3➡k>3➡3<k
両方とも負のとき、k+1<0かつk-3<0➡k<-1かつk<3➡k<-1
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