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(2n+5)(n-3)=________;
(x+3)(-2x-1)=________.
若x=2n+1,y=3·2n+7,则用含x的代数式表示y为
A.y=3x+7
B.y=3x
C.y=3x+8
D.y=3x+4
b2mn=(________)m=(________)2=(________)2n.
已知2n=a,3n=b,则6n=________.