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