JEE MAIN - Mathematics (2023 - 29th January Evening Shift - No. 10)

Let $$\overrightarrow a = 4\widehat i + 3\widehat j$$ and $$\overrightarrow b = 3\widehat i - 4\widehat j + 5\widehat k$$. If $$\overrightarrow c $$ is a vector such that $$\overrightarrow c .\left( {\overrightarrow a \times \overrightarrow b } \right) + 25 = 0,\overrightarrow c \,.(\widehat i + \widehat j + \widehat k) = 4$$, and projection of $$\overrightarrow c $$ on $$\overrightarrow a $$ is 1, then the projection of $$\overrightarrow c $$ on $$\overrightarrow b $$ equals :
$$\frac{3}{\sqrt2}$$
$$\frac{1}{\sqrt2}$$
$$\frac{1}{5}$$
$$\frac{5}{\sqrt2}$$

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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