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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Trigonometric Equations

NDA 2025 · Mathematics · Trigonometric Equations

For the following two (02) items: Let (2x + 3y) = (1)/(2) and (3x +...

For the following two (02) items:
Let cos⁡(2x+3y)=12\cos(2x + 3y) = \dfrac{1}{2} and cos⁡(3x+2y)=32\cos(3x + 2y) = \dfrac{\sqrt{3}}{2}, where −π<(2x+3y)<π-\pi < (2x + 3y) < \pi and −π<(3x+2y)<π-\pi < (3x + 2y) < \pi.

How many values does (x+y)(x + y) have?

Options

  1. ATwo
  2. BThree
  3. CFour
  4. DMore than four

Correct answer

C. Four

Step-by-step solution

Let 2x+3y=u2x + 3y = u and 3x+2y=v3x + 2y = v.

Given cos⁡u=12\cos u = \dfrac{1}{2} and −π<u<π-\pi < u < \pi, the possible values of uu are π3\dfrac{\pi}{3} and −π3-\dfrac{\pi}{3}.

Given cos⁡v=32\cos v = \dfrac{\sqrt{3}}{2} and −π<v<π-\pi < v < \pi, the possible values of vv are π6\dfrac{\pi}{6} and −π6-\dfrac{\pi}{6}.

Adding the two equations, we get u+v=5(x+y)⇒x+y=u+v5u + v = 5(x + y) \Rightarrow x + y = \dfrac{u + v}{5}.

The possible values of u+vu + v are:

π3+π6=π2\dfrac{\pi}{3} + \dfrac{\pi}{6} = \dfrac{\pi}{2}

π3−π6=π6\dfrac{\pi}{3} - \dfrac{\pi}{6} = \dfrac{\pi}{6}

−π3+π6=−π6-\dfrac{\pi}{3} + \dfrac{\pi}{6} = -\dfrac{\pi}{6}

−π3−π6=−π2-\dfrac{\pi}{3} - \dfrac{\pi}{6} = -\dfrac{\pi}{2}

Since all four values of u+vu + v are distinct, there are exactly four distinct values for x+yx + y.
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