All Exams Test series for 1 year @ ₹349 only
Question

If \(\cos\alpha + \cos\beta = 0 = \sin\alpha + \sin\beta\), \(\alpha \ne \beta\) then what is a value of \(\cos 2\alpha + \cos 2\beta + 2 \cos(\alpha + \beta)\) ?

This question was previously asked in
NDA 1 2026 GAT Question Paper (12-Apr-2026)
The correct answer is
0

To solve this problem, we are given that:

  • \(\cos\alpha + \cos\beta = 0\)
  • \(\sin\alpha + \sin\beta = 0\)

We need to find the value of \(\cos 2\alpha + \cos 2\beta + 2 \cos(\alpha + \beta)\).

From the given equations, we can deduce the following trigonometric identities. If \(\cos\alpha + \cos\beta = 0\), then:

\(\cos\alpha = -\cos\beta\)

Similarly, from \(\sin\alpha + \sin\beta = 0\), it follows that:

\(\sin\alpha = -\sin\beta\)

By using the sum-to-product identities for trigonometric functions, the following holds true:

\(\cos(\alpha + \beta) = \cos\alpha\cos\beta - \sin\alpha\sin\beta\)

Since \(\cos\alpha = -\cos\beta\) and \(\sin\alpha = -\sin\beta\), we obtain:

\(\cos(\alpha + \beta) = (-\cos\beta)\cos\beta - (-\sin\beta)\sin\beta\)

\(= -\cos^2\beta + \sin^2\beta\)

\(= -1\) (since \(\cos^2\beta + \sin^2\beta = 1\))

We must find:

\(\cos 2\alpha + \cos 2\beta + 2 \cos(\alpha + \beta)\)

Recall that \(\cos 2\theta = 2\cos^2\theta - 1\), so:

\(\cos 2\alpha = 2\cos^2\alpha - 1\) and \(\cos 2\beta = 2\cos^2\beta - 1\)

Thus:

\(\cos 2\alpha + \cos 2\beta = (2\cos^2\alpha - 1) + (2\cos^2\beta - 1)\)

\(= 2(\cos^2\alpha + \cos^2\beta) - 2\)

Combining the identities, we find:

\(\cos 2\alpha + \cos 2\beta + 2 \cos(\alpha + \beta)\)

\(= (2(\cos^2\alpha + \cos^2\beta) - 2) + 2(-1)\)

\(= 2(\cos^2\alpha + \cos^2\beta) - 4\)

Recall \(\cos^2\alpha + \sin^2\alpha = 1\), and similarly \(\cos^2\beta + \sin^2\beta = 1\), thus:

\(\cos^2\beta = \frac{1}{2}\) and \(\cos^2\alpha = \frac{1}{2}\)

Thus, solving further:

\(2(\frac{1}{2} + \frac{1}{2}) - 4 = 0\)

Final conclusion: The value of \(\cos 2\alpha + \cos 2\beta + 2 \cos(\alpha + \beta)\) is 0.

Was this answer helpful?

Similar Questions

  1. If $x = \sec\theta - \tan\theta$ and $y = \text{cosec}\theta + \cot\theta$, then which one of the following is correct ?

Important Questions from Trigonometric Ratios and Identities

  1. The value of 4 sin 230° + 3 cot 260° - 2 tan 245° is:  

  2. The value of 1 - sin 35° cos 55° is equal to:

  3. If sin 3 θ = cos ( θ – 6°), then  θ is:

  4. If θ = 45°, then what will be the value of  \(\frac{{\\sin \,\theta \, + \,\cos \,\theta }}{{\sin \,\theta \, - \,\cos \,\theta }}\) ?

  5. If sin A = \(\frac{1}{2}\)  and cos B =  \(\frac{1}{2}\)  then find A + B.

Need Expert Advice?
Upcoming Exams
NDA
September 13, 2026
CDS
September 13, 2026
Test Series
NDA img
Defence
NDA 2026 Mock Test Series (Latest Pattern)
501 Tests 1 Tests Free
658 Attempts
4.7(120)
English, Hindi

Start Your Preparation with Prepp Mobile App

Download the app from Google Play & App Store
Download the app from Google Play & App Store
Prepp Mobile App