T(r+1)=C(7,r)x^(7-r)*(2y)^r
设第r+1 项系数最大.
则:C(7,r)2^r≥C(7,r-1)*2^(r-1)
C(7,r)*2^r≥C(7,r+1)*2^(r+1)
故系数最大的项是:T6=672x^2y^5