Question

高一数学 平面向量 已知a的绝对值＝2，b的绝对值＝5，a×b＝－3，求a+b的绝对值？a－b的绝对值？谢谢！

Answer 1

（a+b）^2=a的绝对值^2+2a×b+b的绝对值^2=4-6+25=23
那么a+b的绝对值=根号下23
（a-b）^2=a的绝对值^2-2a×b+b的绝对值^2=4+6+25=35
那么a-b的绝对值=根号下35

Answer 2

|a| = 2
|b| = 5

a×b is a vector , I suppose to be (a.b)
and -3 cannot be a magnitude and I change it as 3

a.b = 3
|a+b|^2 = (a+b).(a+b)
= |a|^2 +2a.b +|b|^2
= 4 + 6 + 25
= 35
|a+b| = 开方35