Cho ba điểm A, B, C. Chứng minh \(3\left( {\overrightarrow {AB} + 2\overrightarrow {BC} } \right) - 2\left( {\overrightarrow {AB} + 3\overrightarrow {BC} } \right) = \overrightarrow {AB} \)
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NV
Nguyễn Việt Lâm
Giáo viên
11 tháng 10 2020
\(BC=AD=\sqrt{AC^2-AB^2}=2a\)
a/ \(T=\left|3\overrightarrow{AB}-4\overrightarrow{BC}\right|\Rightarrow T^2=9AB^2+16BC^2-24\overrightarrow{AB}.\overrightarrow{BC}\)
\(=9a^2+64a^2=73a^2\Rightarrow T=a\sqrt{73}\)
b/ \(T^2=4AB^2+9BC^2+12.\overrightarrow{BA}.\overrightarrow{BC}=4AB^2+9BC^2=40a^2\)
\(\Rightarrow T=2a\sqrt{10}\)
c/ \(T=\left|\overrightarrow{AD}+3\overrightarrow{BC}\right|=\left|\overrightarrow{AD}+3\overrightarrow{AD}\right|=\left|4\overrightarrow{AD}\right|=4AD=8a\)
d/ \(T=\left|2\overrightarrow{DC}-3\overrightarrow{DC}\right|=\left|-\overrightarrow{DC}\right|=CD=AB=a\)
AH
Akai Haruma
Giáo viên
15 tháng 7 2017
Lời giải:
Ta có:
\(\overrightarrow{AI}+\overrightarrow{FA}=\overrightarrow{AB}+\overrightarrow{BI}+\overrightarrow{FD}+\overrightarrow{DA}=\overrightarrow{AB}+\overrightarrow{DA}+\frac{1}{2}(\overrightarrow{BC}+\overrightarrow{CD})\)
Suy ra \(\text{VT}=4(\overrightarrow{AB}+\overrightarrow{DA})+\overrightarrow{BC}+\overrightarrow{CD}\)
\(\Leftrightarrow{VT}=3\overrightarrow{DB}+\overrightarrow{AB}+\overrightarrow{DA}+\overrightarrow{BC}+\overrightarrow{CD}\)
\(\Leftrightarrow{VT}=3\overrightarrow{DB}+(\overrightarrow{AB}+\overrightarrow{BC})+(\overrightarrow{CD}+\overrightarrow{DA})\)
\(\Leftrightarrow{VT}=3\overrightarrow{DB}+\overrightarrow{AC}+\overrightarrow{CA}=3\overrightarrow{DB}=\text{VP}\)
Ta có đpcm
27 tháng 2 2016
Do tam giác ABC vuông tại A và \(\widehat{B}=30^o\) \(\Rightarrow C=60^o\)
\(\Rightarrow\left(\overrightarrow{AB},\overrightarrow{BC}\right)=150^o;\)\(\left(\overrightarrow{BA},\overrightarrow{BC}\right)=30^o;\left(\overrightarrow{AC},\overrightarrow{CB}\right)=120^o\)
\(\left(\overrightarrow{AB},\overrightarrow{AC}\right)=90^o;\left(\overrightarrow{BC},\overrightarrow{BA}\right)=30^o\).Do vậy:
a) \(\cos\left(\overrightarrow{AB},\overrightarrow{BC}\right)+\sin\left(\overrightarrow{BA},\overrightarrow{BC}\right)+\tan\frac{\left(\overrightarrow{AC},\overrightarrow{CB}\right)}{2}\)
\(=\cos150^o+\sin30^o+\tan60^o\)
\(=-\frac{\sqrt{3}}{2}+\frac{1}{2}+\sqrt{3}\)
\(=\frac{\sqrt{3}+1}{2}\)
b) \(\sin\left(\overrightarrow{AB},\overrightarrow{AC}\right)+\cos\left(\overrightarrow{BC},\overrightarrow{AB}\right)+\cos\left(\overrightarrow{CA},\overrightarrow{BA}\right)\)
\(=\sin90^o+\cos30^o+\cos0^o\)
\(=1+\frac{\sqrt{3}}{2}\)
\(=\frac{2+\sqrt{3}}{2}\)
HP
15 tháng 12 2020
Có vẻ không đúng.
Giả sử \(\overrightarrow{AB}+\overrightarrow{MB}+\overrightarrow{MA}=\overrightarrow{0}\)
\(\Leftrightarrow\overrightarrow{MB}+\left(\overrightarrow{MA}+\overrightarrow{AB}\right)=\overrightarrow{0}\)
\(\Leftrightarrow\overrightarrow{MB}+\overrightarrow{MB}=\overrightarrow{0}\)
\(\Leftrightarrow2\overrightarrow{MB}=\overrightarrow{0}\)
\(\Leftrightarrow M\equiv B\) (Vô lí)
17 tháng 8 2019
a) Chữa đề: \(\overrightarrow{CA}+\overrightarrow{DB}=\overrightarrow{CB}+\overrightarrow{DA}=2\overrightarrow{NM}\)
\(Ta\text{ }có:\overrightarrow{CA}+\overrightarrow{DB}=\overrightarrow{CB}+\overrightarrow{BA}+\overrightarrow{DA}+\overrightarrow{AB}\\ =\overrightarrow{CB}+\overrightarrow{DA}+\left(\overrightarrow{BA}+\overrightarrow{AB}\right)=\overrightarrow{CB}+\overrightarrow{DA}\)
\(\)\(\overrightarrow{CA}+\overrightarrow{DB}=\overrightarrow{CA}+\overrightarrow{CB}+\overrightarrow{DC}\\ =2\overrightarrow{CM}+2\overrightarrow{NC}=2\left(\overrightarrow{NC}+\overrightarrow{CM}\right)=2\overrightarrow{NM}\)
Vậy \(\overrightarrow{CA}+\overrightarrow{DB}=\overrightarrow{CB}+\overrightarrow{DA}=2\overrightarrow{NM}\)
\(\text{b) }\overrightarrow{AD}+\overrightarrow{BD}+\overrightarrow{AC}+\overrightarrow{BC}=-\left(\overrightarrow{DA}+\overrightarrow{DB}+\overrightarrow{CA}+\overrightarrow{CB}\right)\\ =-\left[\left(\overrightarrow{DA}+\overrightarrow{DB}\right)+\left(\overrightarrow{CA}+\overrightarrow{CB}\right)\right]\\ =-\left(2\overrightarrow{DM}+2\overrightarrow{CM}\right)=2\left(\overrightarrow{MD}+\overrightarrow{MC}\right)=4\left(\overrightarrow{MN}\right)\)
\(\text{c) }2\left(\overrightarrow{AB}+\overrightarrow{AI}+\overrightarrow{NA}+\overrightarrow{DA}\right)\\ =2\left[\left(\overrightarrow{AB}+\overrightarrow{DA}\right)+\left(\overrightarrow{AI}+\overrightarrow{NA}\right)\right]\\ =2\left[\left(\overrightarrow{AB}+\overrightarrow{BA}+\overrightarrow{DB}\right)+\overrightarrow{NI}\right]=2\left(\overrightarrow{DB}+\overrightarrow{NI}\right)\)
Mà IN là dường trung bình \(\Delta BCD\)
\(\Rightarrow\left\{{}\begin{matrix}IN//BD\\IN=\frac{1}{2}BD\end{matrix}\right.\Rightarrow\overrightarrow{IN}=\frac{1}{2}\overrightarrow{BD}\\ \Rightarrow2\left(\overrightarrow{AB}+\overrightarrow{AI}+\overrightarrow{NA}+\overrightarrow{DA}\right)\\ =2\left(\overrightarrow{DB}+\overrightarrow{NI}\right)=2\left(\overrightarrow{DB}+\frac{1}{2}\overrightarrow{DB}\right)=2\cdot\frac{3}{2}\overrightarrow{DB}=3\overrightarrow{DB}\)
NV
Nguyễn Việt Lâm
Giáo viên
26 tháng 11 2021
\(\left|\overrightarrow{MA}+\overrightarrow{BC}+\overrightarrow{AB}\right|=\left|\overrightarrow{AC}+\overrightarrow{BA}\right|\)
\(\Leftrightarrow\left|\overrightarrow{MA}+\overrightarrow{AC}\right|=\left|\overrightarrow{BC}\right|\)
\(\Leftrightarrow\left|\overrightarrow{MC}\right|=\left|\overrightarrow{BC}\right|\)
\(\Leftrightarrow MC=BC\)
\(\Rightarrow\) Tập hợp M là đường tròn tâm C bán kính BC
Ta có: \(3\left( {\overrightarrow {AB} + 2\overrightarrow {BC} } \right) - 2\left( {\overrightarrow {AB} + 3\overrightarrow {BC} } \right)\)\( = 3\overrightarrow {AB} + 3.\left( {2\overrightarrow {BC} } \right) - \left[ {2\overrightarrow {AB} + 2.\left( {3\overrightarrow {BC} } \right)} \right]\)
\[ = 3\overrightarrow {AB} + 6.\overrightarrow {BC} - \left( {2\overrightarrow {AB} + 6.\overrightarrow {BC} } \right)\]\[ = 3\overrightarrow {AB} + 6.\overrightarrow {BC} - 2\overrightarrow {AB} - 6.\overrightarrow {BC} \]
\[ = \left( {3\overrightarrow {AB} - 2\overrightarrow {AB} } \right) + \left( {6.\overrightarrow {BC} - 6.\overrightarrow {BC} } \right) = \overrightarrow {AB} .\]