JEE MAIN - Mathematics (2023 - 29th January Evening Shift - No. 10)
Explanation
$$[\matrix{ {\overrightarrow c } & {\overrightarrow a } & {\overrightarrow b } \cr } ] = - 25$$
Let $$\overrightarrow c = l\widehat i + n\widehat j + n\widehat k$$
$$\left| {\matrix{ l & m & n \cr 4 & 3 & 0 \cr 3 & { - 4} & 5 \cr } } \right| = - 25$$
$$ \Rightarrow 3l - 4m - 5n = - 5$$ ..... (i)
$$\overrightarrow c \,.\,(\widehat i + \widehat j + \widehat k) = 4$$
$$ \Rightarrow l + m + n = 4$$ ..... (ii)
$${{\overrightarrow c \,.\,\overrightarrow a } \over {|\overrightarrow a |}} = 1 \Rightarrow \overrightarrow c \,.\,\overrightarrow a = 5$$
$$ \Rightarrow 4l + 3m = 5$$ ...... (iii)
Using (i), (ii) and (iii)
$$l = 2,m = - 1,n = 3$$
Now, $${{\overrightarrow c \,.\,\overrightarrow b } \over {|\overrightarrow b |}} = {{25} \over {5\sqrt 2 }} = {5 \over {\sqrt 2 }}$$
$$\therefore$$ Option (2) is correct.
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