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Solve[{8 Cos[x]^3 - 4 Cos[x]^2 - 4 Cos[x] + 1 == 0, \[Pi] > x > 0}, {x}] // FullSimplify
{{x -> (5 \[Pi])/7}, {x -> (3 \[Pi])/7}, {x -> \[Pi]/7}}
Solve[{16 Cos[x]^4 - 8 Cos[x]^3 - 12 Cos[x]^2 + 4 Cos[x] + 1 == 0, \[Pi] > x > 0}, {x}] // FullSimplify
{{x -> \[Pi]/3}, {x -> (7 \[Pi])/9}, {x -> (5 \[Pi])/9}, {x -> \[Pi]/9}}
Solve[{32 Cos[x]^5 - 16 Cos[x]^4 - 32 Cos[x]^3 + 12 Cos[x]^2 + 6 Cos[x] - 1 == 0, \[Pi] > x > 0}, {x}]
{{x -> (9 \[Pi])/11}, {x -> (7 \[Pi])/11}, {x -> (5 \[Pi])/11}, {x -> (3 \[Pi])/11}, {x -> \[Pi]/11}}
Solve[{64 Cos[x]^6 - 32 Cos[x]^5 - 80 Cos[x]^4 + 32 Cos[x]^3 + 24 Cos[x]^2 - 6 Cos[x] - 1 == 0
{{x -> (11\[Pi])/13}, {x -> (9\[Pi])/13}, {x -> (7\[Pi])/13}, {x -> (5\[Pi])/13}, {x -> (3\[Pi])/13}, {x ->\[Pi]/13}}
Solve[{128 Cos[x]^7 - 64 Cos[x]^6 - 192 Cos[x]^5 + 80 Cos[x]^4 + 80 Cos[x]^3 - 24 Cos[x]^2 - 8 Cos[x] + 1 == 0, \[Pi] > x > 0}, {x}] // FullSimplify
{{x->\[Pi]/3},{x->(3\[Pi])/5},{x->\[Pi]/5},{x->(13\[Pi])/15},{x->(11\[Pi])/15},{x->(7\[Pi])/15},{x->\[Pi]/15}}
Solve[{256 Cos[x]^8 - 128 Cos[x]^7 - 448 Cos[x]^6 + 192 Cos[x]^5 + 240 Cos[x]^4 - 80 Cos[x]^3 - 40 Cos[x]^2 + 8 Cos[x] + 1 == 0, \[Pi] > x > 0}, {x}] // FullSimplify
{{x -> (15 \[Pi])/17}, {x -> (13 \[Pi])/17}, {x -> (11 \[Pi])/17}, {x -> (9 \[Pi])/17}, {x -> (7 \[Pi])/17}, {x -> (5 \[Pi])/17}, {x -> (3 \[Pi])/17}, {x -> \[Pi]/17}}
Solve[{512 Cos[x]^9 - 256 Cos[x]^8 - 1024 Cos[x]^7 + 448 Cos[x]^6 + 672 Cos[x]^5 - 240 Cos[x]^4 - 160 Cos[x]^3 + 40 Cos[x]^2 + 10 Cos[x] - 1 == 0, \[Pi] > x > 0}, {x}] // FullSimplify
{{x -> (17 \[Pi])/19}, {x -> (15 \[Pi])/19}, {x -> (13 \[Pi])/19}, {x -> (11 \[Pi])/19}, {x -> (9 \[Pi])/19}, {x -> (7 \[Pi])/19}, {x -> (5 \[Pi])/19}, {x -> (3 \[Pi])/19}, {x -> \[Pi]/19}}
Solve[{1024 Cos[x]^10 - 512 Cos[x]^9 - 2304 Cos[x]^8 + 1024 Cos[x]^7 + 1792 Cos[x]^6 - 672 Cos[x]^5 - 560 Cos[x]^4 +160 Cos[x]^3 + 60 Cos[x]^2 - 10 Cos[x] - 1 == 0, \[Pi] > x > 0}, {x}]
{{x -> \[Pi]/3}, {x -> (5 \[Pi])/7}, {x -> (3 \[Pi])/7}, {x -> \[Pi]/7}, {x -> (19 \[Pi])/21}, {x -> (17 \[Pi])/21}, {x -> (13 \[Pi])/21}, {x -> (11 \[Pi])/21}, {x -> (5 \[Pi])/21}, {x -> \[Pi]/21}}
Solve[{2048 Cos[x]^11 - 1024 Cos[x]^10 - 5120 Cos[x]^9 + 2304 Cos[x]^8 + 4608 Cos[x]^7 - 1792 Cos[x]^6 - 1792 Cos[x]^5 + 560 Cos[x]^4 + 280 Cos[x]^3 - 60 Cos[x]^2 - 12 Cos[x] + 1 == 0,
{{x -> (21 \[Pi])/23}, {x -> (19 \[Pi])/23}, {x -> (17 \[Pi])/23}, {x -> (15 \[Pi])/23}, {x -> (13 \[Pi])/23}, {x -> (11 \[Pi])/23}, {x -> (9 \[Pi])/23}, {x -> (7 \[Pi])/23}, {x -> (5 \[Pi])/23}, {x -> (3 \[Pi])/23}, {x -> \[Pi]/23}} |
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