Verified Answer :

bilaal

use cos (2∅)= 2cos² ∅ -1

5(2cos²∅ -1 )=1
10cos²∅ -5 = 1
10cos²∅ = 6
cos²∅ = 0.6
cos∅ = ± √(0.6)
∅=  39.2,180 - 39.2, 180 + 39.2 , 360 - 39.2
∅ = 39.2 , 140.8 , 219.2 , 320.8 

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