((3a+b)²-(a+3b)²)×2ab=((3a+b-a-3b)(3a+b+a+3b))×2ab=((2a-2b)(4a+4b))×2ab=(2(a-b)×4(a+b))×2ab=(8(a-b)(a+b))×2ab=(8(a²-b²))×2ab=(8a²-8b²)×2ab=16a³b-16ab³
((x²+2x)²+(2x²-x)²):(5x²)=(x²+2x)²+(2x²-x)²/5x²=x⁴+4x³+4x²+4x⁴-4x³+x²/5x²=5x⁴+5x²/5x²=5x²(x²+1)/5x²=x²+1