一般項は
20Cr*(2x^2)^(20-r)*(-1/x)^r
=20Cr*2^(20-r)*(-1)^r*x^(40-3r)
条件を満たすのは
40-3r=-14
r=18
よって係数は、
20C18*2^(20-18)*(-1)^18
=760
だと思います
間違っていたらすみません(´･ω･`)
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