(2a-3b)·(2a+b)
=4a²-6ab+2ab-3b²
=4a²-4ab-3b²
=4*16-4ab-3*9
=61
ab=-6
a(a+b)
=a²+ab
=16-6
=10
|a+b|
=√(a+b)²
=√(a²+2ab+b²)
=√(16-12+9)
=√13
向量a在向量a+b方向上的投影
=a(a+b)/|a+b|
=10/√13
=10√13/13