用排列组合来解,即为在9个数里面取出2个数,再在剩余的7个数里面取出3个数,再在剩余的4个数中取出的4个数的组合数
C(9,2)*C(7,3)*C(4,4)=1260
验证：
(a+b+c)^9=72*a*b*c^7+252*a^2*b*c^6+630*a^4*b^4*c+1260*a^4*b^3*c^2+252*a*b^2*c^6+504*a*b^3*c^5+252*a*b^6*c^2+504*a*b^5*c^3+1260*a^2*b^3*c^4+1260*a^3*b^2*c^4+504*a^5*b*c^3+756*a^2*b^2*c^5+504*a^5*b^3*c+504*a^3*b^5*c+756*a^5*b^2*c^2+1260*a^4*b^2*c^3+756*a^2*b^5*c^2+72*a*b^7*c+252*a^2*b^6*c+504*a^3*b*c^5+630*a^4*b*c^4+252*a^6*b^2*c+72*a^7*b*c+1260*a^2*b^4*c^3+1260*a^3*b^4*c^2+1680*a^3*b^3*c^3+252*a^6*b*c^2+630*a*b^4*c^4+a^9+9*a^8*b+9*a^8*c+36*a^7*b^2+36*a^7*c^2+84*a^6*b^3+84*a^6*c^3+b^9+c^9+126*a^5*b^4+126*a^5*c^4+126*a^4*b^5+126*a^4*c^5+84*a^3*b^6+84*a^3*c^6+36*a^2*b^7+36*a^2*c^7+9*a*b^8+9*a*c^8+9*b^8*c+36*b^7*c^2+84*b^6*c^3+126*b^5*c^4+126*b^4*c^5+84*b^3*c^6+36*b^2*c^7+9*b*c^8
a^2*b^3*c^4得系数为1260