Q35 / 60 · JEE Advanced 2014 (Paper 1) · Mathematics

Let a→, b→ and c→ be three non - coplanar unit vectors such that the angle between every pair of them is π3. If a→ × b→+ b→ × c→=pa→+qb→+rc→ , where p, q and r are scalars, then the value of p2+2q2+r2q2  is _______

Correct answer: 4

Solution

a→=b→= c→=1
a→ × b→+ b→ × c→=pa→+qb→+rc→
a→ . b→ × c→=p+q a→ . b→+ra→ . c→
And a→  b→  c→=12
p+q2+r2=a→ b→ c→ .......(i)
p2+q+r2=0 ......(ii)
p2+q2+r=a→ b→ c→ ......(iii)
⇒ p=r= -q
p2+2q2+r2q2=4

Hence, the correct answer is 4.

Relevant Questions