b均为向量 |a|=2 |b|=3 (2a-3b)·(2a+b)=3 求①a·b ②|a+b|

(2a-3b)·(2a+b)=3 = 4a·a -4a·b -3b·b = 4* 2^2 -4a·b - 3* 3^2 = -4a·b - 11, a·b = -7/2|a+b| =根号[(a+b)·(a+b)] = 根号[a·a +2a·b +b·b] = 根号(2^2 -7 + 3^2)= 根号6