Question

已知向量A的模=2向量B的模=3,向量A与向量B的夹角为60度,则丨2a-3b丨等于

Answer 1

解：|a|=2,|b|=3. |a-b|=√(a-b)^2 =√(a^2-2ab+b^2). =√(2^2-2|a||b|cos<a,b>+b^2). =√(4-2*2*3*cos60°+3^2). =√(4-6+9). =√7. |a-2b|=√[a^2-2a*2b+(2b)^2]. =√(a^2-2*2ab+4b^2). =√(a^2-2*2|a||b|cos<a,b>+4b^2). =√[4-4*2*3*(1/2)+4^9]. =√28. =2√7.

追答

解：|a|=2,|b|=3.
|a-b|=√(a-b)^2
=√(a^2-2ab+b^2).
=√(2^2-2|a||b|cos+b^2).
=√(4-2*2*3*cos60°+3^2).
=√(4-6+9).
=√7.
|a-2b|=√[a^2-2a*2b+(2b)^2].
=√(a^2-2*2ab+4b^2).
=√(a^2-2*2|a||b|cos+4b^2).
=√[4-4*2*3*(1/2)+4^9].
=√28.
=2√7.