Related Formula
2θ = 2 ²θ - 1$$\cos 2\theta = 2\cos^2\theta - 1$$
Core Logic
Substitute the double-angle formula into the given equation to form a quadratic in θ$\cos\theta$:
√(3)(2 ²θ - 1) + 8 θ + 3√(3) = 0$$\sqrt{3}(2\cos^2\theta - 1) + 8\cos\theta + 3\sqrt{3} = 0$$
2√(3) ²θ + 8 θ + 2√(3) = 0$$2\sqrt{3}\cos^2\theta + 8\cos\theta + 2\sqrt{3} = 0$$
Step 1: Solve the Quadratic
Factorize the quadratic equation:
2√(3) ²θ + 2 θ + 6 θ + 2√(3) = 0$$2\sqrt{3}\cos^2\theta + 2\cos\theta + 6\cos\theta + 2\sqrt{3} = 0$$
Wait, 2 × 2√(3) = 12$2 \times 2\sqrt{3} = 12$. The factors of 12$12$ that sum to 8$8$ are 6$6$ and 2$2$.
2 θ(√(3) θ + 1) + 2√(3)(√(3) θ + 1) = 0$$2\cos\theta(\sqrt{3}\cos\theta + 1) + 2\sqrt{3}(\sqrt{3}\cos\theta + 1) = 0$$
(√(3) θ + 1)(2 θ + 2√(3)) = 0$$(\sqrt{3}\cos\theta + 1)(2\cos\theta + 2\sqrt{3}) = 0$$
This gives:
θ = - 1√(3) or θ = -√(3)$$\cos\theta = -\frac{1}{\sqrt{3}} \quad \text{or} \quad \cos\theta = -\sqrt{3}$$
Since -1 ≤ θ ≤ 1$-1 \leq \cos\theta \leq 1$, we reject θ = -√(3)$\cos\theta = -\sqrt{3}$.
Step 2: Count Solutions in Interval
We need solutions for θ = - 1√(3)$\cos\theta = -\frac{1}{\sqrt{3}}$ in the interval [-3π, 2π]$[-3\pi, 2\pi]$.
The period of cosine is 2π$2\pi$. The equation θ = k$\cos\theta = k$ (where -1 < k < 0$-1 < k < 0$) has 2$2$ solutions per 2π$2\pi$ interval.
Intervals:
[0, 2π]$[0, 2\pi]$: 2$2$ solutions (in Quadrants II and III).
[-2π, 0]$[-2\pi, 0]$: 2$2$ solutions.
[-3π, -2π]$[-3\pi, -2\pi]$: 1$1$ solution (in Quadrant II equivalent, which is Quadrant III when going backwards. Specifically, from -3π$-3\pi$ to -2π$-2\pi$ covers the top half of the circle. Wait, [-3π, -2π]$[-3\pi, -2\pi]$ goes from 180^°$180^\circ$ to 360^°$360^\circ$ logically, i.e., quadrants III and IV. $\cos$ is negative in Quadrant III. So exactly 1$1$ solution).
Total solutions = 2 + 2 + 1 = 5$2 + 2 + 1 = 5$.
Pattern Recognition
Mapping phase intervals chunk by chunk (2π$2\pi$ cycles yield 2$2$ roots for | x|<1$|\cos x|<1$) prevents overcounting when domain bounds don't cleanly align with full periods.
Chapter Mix
Class 11 Maths: Trigonometric Functions