Câu 8:
Cho a→, b→\overrightarrow a ,\,\overrightarrow ba,b có ∣a→∣=4, ∣b→∣=5, (a→,b→)=60∘.\left| {\overrightarrow a } \right| = 4,\,\left| {\overrightarrow b } \right| = 5,\,\left( {\overrightarrow a ,\overrightarrow b } \right) = 60^\circ .a=4,b=5,(a,b)=60∘. Tính ∣a→−5b→∣.\left| {\overrightarrow a - 5\overrightarrow b } \right|.a−5b.
