# Solve this: Q. Which one of the following vectors of magnitude $\sqrt{51}$ makes equal angles with three vectors                     ?

• 1
Hi Kapoor, 5 i - j - 5k will be the right one
Because angle in cosine form ie. Cos @ = x vec . y vec / |x| |y|
|a| = 1 and |b| = 1 and |c| = 1
So as we take the a vec along with our right answer then cos @ = (5 + 2 - 10) /3 * ./51 = -1/./51
And with b vec, -20 + 15 / 5 *./51 = -1/ ./51
Finally with c vec = - 1/ ./51
At the outset mod of option d is not ./51. So it could be deleted at the beginning itself

• -1
What are you looking for?