求(2x-3y)^9展开式中各项系数绝对值的和。要求方法，谢谢！

(2x-3y)^9 =2^9 .( x- (3/2)y)^9 consider (1+x)^n = nC0 +nC1x+nC2x^2+....+nCn. x^n x= 2/3 (1+3/2)^9 = nC0 +nC1.(3/2) +nC2.(3/2)+....+nCn. (3/2)^n 5^9/2^9 = nC0 +nC1.(3/2) +nC2.(3/2)+....+nCn. (3/2)^n -------