2 cosA=a+1/a, prove that : cos3A=1/2(a^3 +1/a^3). plz solve this.
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Answer:
Step-by-step explanation:
//Remember: Cos3A = 4Cos³A - 3CosA
4Cos³A - 3CosA = = 4 [1/2(a+1/a)]³ - 3[1/2(a+1/a)]
=> 4/8 (a+1/a)³ - 3/2 (a+1/a)
=> 1/2(a³+1/a³+ 3(a+1/a) - 3/2(a+1/a)
=> 1/2(a³+1/a³) + 3/2(a+1/a) - 3/2(a+1/a)
=> 1/2(a³+1/a³)
=> R.H.S.
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