(3a²-b²)-3(a²-2b²)-3a³-[2x²-(5x+1)]

(3a²-b²)-3(a²-2b²)-3a³-[2x²-(5x+1)]
(3a²-b²)-3(a²-2b²)
-3a³-[2x²-(5x+1)]

(3a²-b²)-3(a²-2b²)-3a³-[2x²-(5x+1)]
(3a²-b²)-3(a²-2b²)
=3a²-b²-3a²+6b²
=5b²
-3a³-[2x²-(5x+1)]
=-3a³-2x²+5x+1
括号外面是+号,去括号时括号内的符合不变,
括号外面是-号,去括号时括号内的符合要改变.

我也不会合并同类项的题