曲线x^2/25+y^2/9=1与曲线x^2/(25-k)+y^2/(9-k)=1(k

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曲线x^2/25+y^2/9=1与曲线x^2/(25-k)+y^2/(9-k)=1(k

焦点相同
x^2/25+y^2/9=1
c^2=25-9=16
x^2/(25-k)+y^2/(9-k)=1
C'^2=(25-k)-(9-k)=16
所以选2