已知A=5a^3b+2a^4-3a^2b-ab^3,B=6ab^3-8a^3b+3a^4-5b^4,求2(2A+B)-3(A+B)的值是多
问题描述：
已知A=5a^3b+2a^4-3a^2b-ab^3,B=6ab^3-8a^3b+3a^4-5b^4,求2(2A+B)-3(A+B)的值是多
答
2(2A + B) -3(A + B)
= 4A + 2B - 3A - 3B
= A - B
= (5a^3b + 2a^4 - 3a^2b - ab^3) - (6ab^3 - 8a^3b + 3a^4 - 5b^4)
= 5a^3b + 2a^4 - 3a^2b - ab^3 - 6ab^3 + 8a^3b - 3a^4 + 5b^4
= 13a^3b - a^4 - 3a^2b - 7ab^3 + 5b^4