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Solve: 2 cos2x + 3 sinx = 0

Answer»

We have:

2 cos2x + 3 sinx = 0

2 cos2x= – 3 sinx

Squaring both sides, we get 

4 cos4x = 9 sin2x

4 cos4x – 9 (1 – cos2x) = 0

Let cos2x = y

4y2 + 9y – 9 = 0

4y2 + 12y – 3y – 9 = 0

4y (y + 3) – 3 (y + 3) = 0

(y + 3) (4y – 3) = 0

y = – 3 or, y = \(\frac{3}{4}\)

cos2x = – 3 or cos2x = \(\frac{3}{4}\)

∴  cos2x = \(\frac{3}{4}\) = cos2\((\frac{\pi}{6})\) 

x = nπ ±  \(\frac{\pi}{6}\)



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