Question

已知向量a的模=向量b的模=2 ﹙a+2b﹚·﹙a-b﹚= -2 求向量a与向量b的夹角为多少度,

Answer 1

|a|=|b|=2

﹙a+2b﹚·﹙a-b﹚= -2

2|a|²-4|b|²+2ab=2|a|²-4|b|²+2|a||b|cos@=-2

cos@=0.75