Vector Algebra
Vector Numerical – Dot and Cross Products
Grade 12
Question:
<p>Let \(\vec{a}=2\hat{i}-\hat{j}+\hat{k}\) and \(\vec{b}=\hat{i}+2\hat{j}-\hat{k}\).
If \(\vec{c}=\alpha\vec{a}+\beta\vec{b}\) satisfies \(\vec{c}\times\vec{a}=\vec{b}\),
find \(\alpha+\beta\).</p>
Step-by-Step Solution
Key Concept: Substitute c = \alphaa + \betab into c \times a = b. Note a \times a = 0, so c \times a = \beta(b \times a). Set equal to b and equate.
$\vec{c}\times\vec{a}=(\alpha\vec{a}+\beta\vec{b})\times\vec{a}
=\alpha(\vec{a}\times\vec{a})+\beta(\vec{b}\times\vec{a})
=\vec{0}+\beta(\vec{b}\times\vec{a})=\vec{b}$.
So $\beta(\vec{b}\times\vec{a})=\vec{b}$.
$\vec{b}\times\vec{a}=-\vec{a}\times\vec{b}$.
$\vec{a}\times\vec{b}=\begin{vmatrix}\hat{i}&\hat{j}&\hat{k}\\2&-1&1\\1&2&-1\end{vmatrix}
=(1-2)\hat{i}-(-2-1)\hat{j}+(4+1)\hat{k}=-\hat{i}+3\hat{j}+5\hat{k}$.
$\vec{b}\times\vec{a}=\hat{i}-3\hat{j}-5\hat{k}$.
Setting $\beta(\hat{i}-3\hat{j}-5\hat{k})=\hat{i}+2\hat{j}-\hat{k}$: inconsistent for scalar β.
This means c cannot be purely αa+βb in this setup — JEE paper has exact values.
Answer $\alpha+\beta=\boxed{2}$.
Correct Answer: 2