问题
解答题
已知|
(Ⅰ)求
(Ⅱ)求|
|
答案
(1)
•a
=|b
||a
|cosθ=2×3×cos120°=-3b
(2)|
+a
|2=b
2+2 a
•a
+b
2b
=9+2|
||a
|cos120°+4b
=13-6
=7
∴|
+a
|=b 7
已知|
(Ⅰ)求
(Ⅱ)求|
|
(1)
•a
=|b
||a
|cosθ=2×3×cos120°=-3b
(2)|
+a
|2=b
2+2 a
•a
+b
2b
=9+2|
||a
|cos120°+4b
=13-6
=7
∴|
+a
|=b 7