∵a-b=5 ab=4
∴1.3a²+3b²
=3(a²+b²)
3[(a²-2ab+b²)+2ab]
=3[(a-b)²+2ab]
=3×(5²+8)
=99
2.(a+b)²
=a²+2ab+b²
=a²-2ab+b²+4ab
=(a-b)²+4ab
=5²+4×4
=414ab怎么来的- -a²+2ab+b²=a²-2ab+b²+4ab原来是2ab2ab=-2ab+4ab