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← JEE Advanced 2021 · Paper 1
MathematicsInteger+4Question 57 of 57

JEE Advanced 2021 · Paper 1, Mathematics question 19

Let u⃗\vec u, v⃗\vec v and w⃗\vec w be vectors in three-dimensional space, where u⃗\vec u and v⃗\vec v are unit vectors which are not perpendicular to each other and
u⃗⋅w⃗=1,v⃗⋅w⃗=1,w⃗⋅w⃗=4\vec u\cdot\vec w = 1,\qquad \vec v\cdot\vec w = 1,\qquad \vec w\cdot\vec w = 4
If the volume of the parallelopiped, whose adjacent sides are represented by the vectors u⃗\vec u, v⃗\vec v and w⃗\vec w, is 2\sqrt2, then the value of ∣3u⃗+5v⃗∣|3\vec u + 5\vec v| is ____.

Answer and worked solution

Official answer: 7

Gram determinant. Let u⃗⋅v⃗=c≠0\vec u\cdot\vec v=c\ne0. The square of the volume is
∣1c1c11114∣=2+2c−4c2=(2)2=2,\begin{vmatrix}1&c&1\\c&1&1\\1&1&4\end{vmatrix}=2+2c-4c^2=(\sqrt2)^2=2,
so c(1−2c)=0c(1-2c)=0 and c=12c=\frac12.

Value.
∣3u⃗+5v⃗∣2=9+25+30c=49,∣3u⃗+5v⃗∣=7.|3\vec u+5\vec v|^2=9+25+30c=49,\qquad|3\vec u+5\vec v|=7.