解：{3x+y=2k+3① {x+3y=5②①-②，得2x-2y=2k-2化简得x-y=k-1∴k=x-y+1∵k≥9∴x-y+1≥9则x-y+1≥8
3x+y=2k+3①x+3y=5②方程①-②有：2x-2y=2k-2∴x-y=k-1∵k≥9∴k-1≥8即x-y≥8
两等式相减。3x+y-(x+3y)=2k+3-5x-y=k-1x-y>=8
解方程x=(6K-4)/8y=(12-2k)/8x-y=k-2K≥9x-y≥7
