雪柳青青
春芽
共回答了17个问题采纳率:88.2% 举报
以下 (a.b) 表示点乘.
= = = = = = = = =
因为 |a+b| =5,
所以 25 =(a+b)^2
=a^2 +2(a.b) +b^2. (1)
因为 |a-b| =5,
所以 25 =(a-b)^2
=a^2 -2(a.b) +b^2. (2)
(1) +(2)得
50 =2(a^2) +2(b^2),
即 b^2 =25 -a^2.
又因为 |a| =3,
所以 b^2 =16,
所以 |b| =4.
= = = = = = = = =
写得有点乱了.
向量模的问题,考虑平方.
1年前
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