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Câu hỏi: Cho \(\overrightarrow a = (x; 2);\overrightarrow b = (- 5; 1);\overrightarrow c = (x; 7)\) . Vectơ \(\overrightarrow c = 2\overrightarrow a + 3\overrightarrow b \) nếu:
a) \(x = -15\) b) \(x = 3\)
c) \(x = 15\) d) \(x = 5\)
a) \(x = -15\) b) \(x = 3\)
c) \(x = 15\) d) \(x = 5\)
Lời giải chi tiết
Ta có: \(\overrightarrow a = (x; 2);\overrightarrow b = (- 5; 1);\overrightarrow c = (x; 7)\) nên:
\(\eqalign{
& \overrightarrow c = 2\overrightarrow a + 3\overrightarrow b = (2x - 15; 7) \cr
& \Rightarrow \left\{ \matrix{
2x - 15 = x \hfill \cr
7 = 7 \hfill \cr} \right. \Leftrightarrow x = 15. \cr} \)
Ta có: \(\overrightarrow a = (x; 2);\overrightarrow b = (- 5; 1);\overrightarrow c = (x; 7)\) nên:
\(\eqalign{
& \overrightarrow c = 2\overrightarrow a + 3\overrightarrow b = (2x - 15; 7) \cr
& \Rightarrow \left\{ \matrix{
2x - 15 = x \hfill \cr
7 = 7 \hfill \cr} \right. \Leftrightarrow x = 15. \cr} \)