If cos4α - sin4α = 5/6 then the value of 2cos2α - 1 = . . . . . . . .

Answer: Option B

cos⁴α - sin⁴α = $$\frac{5}{6}$$
(cos²α + sin²α)(cos²α - sin²α) = $$\frac{5}{6}$$
1 × (cos²α - 1 + cos²α) = $$\frac{5}{6}$$
2cos²α - 1 = $$\frac{5}{6}$$