1: Thinking: If you want to know the included angle, you can only take the product of quantities without coordinates. From A to C, we can only get a*c=0, but we also know the relationship between abc, so we seek the relationship between A and B, and just get rid of this guy C.

Answer: c=a+b c*a=0, so c*a=(a+b)a=0 gives a 2+a * b = 0.

A 2 = 1, so A * B = | A | | B | * Cos angle = 1*2*cos angle =- 1.

The included angle of cos =- 1/2 is 120.

2. Thinking: There are two solutions to this module length problem. 1 is the coordinate, and 2 is the expansion. There are no coordinates here. We choose to expand, and expansion must be squared first.

Answer: | 3a-b | 2 = 9a2+b2-6a * b = 9 * 42+52-6a * b =144+25-6 * | a ||||| b | * cos60 =160.

Finally, don't forget the recipe: Root Sign 109.

Don't ask me anything

Willing to adopt.