Q1 / 100 · NTA Abhyas (JEE Main) 2020 · Mathematics · Vector Algebra

Let V→θ=cos⁡θ+sec⁡θa^+cos⁡θ-secθb^, where a^ and b^ are unit vectors and the angle between a^ and b^ is 60o, then the minimum value of V→4 is equal to

Correct answer: 12

Solution

V→2=cos⁡θ+sec⁡θa^+cos⁡θ-sec⁡θb^2
=cos⁡θ+sec⁡θ2+cos⁡θ-sec⁡θ2+(cos⁡θ-sec⁡θ)(cos⁡θ+sec⁡θ)
=3cos2⁡θ+sec2⁡θ≥23
⇒ least value of V→2=23
⇒ least value of V→4=12

Hence, the correct answer is 12.

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