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cos2α=cos²α-sin²α=2cos²α-1
nowa niewiadoma u=cos2x
2u²-1=5u-4
2u²-5u+3=0
Δ=25-24=1 √Δ=1
u1=(5-1)/2=1
u2=(5+1)/2=3 ∉ D
cos2x=1 ∧ x∈<0,π>
2x=0 ∨ 2x=2π
x1=0 x2=π
x1+x2=π