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# If 2 cosθ = a +1a, prove that 2 cos 2θ = a2 +1a2and2 cos 3θ = a3 +1a3

Vikas TU
14149 Points
4 years ago
Dear Student,
2 cos theta = a+1/a
squaring both sides we get,(2 cos theta)^2=( a+1/a)^2
=>4 cos^2 theta=a^2+1/(a^2)+2
=>4 cos^2 theta=a^2+1/(a^2)+2
=>4 cos^2 theta-2=a^2+1/(a^2)
=>2(2cos^2 theta-1)=a^2+1/(a^2)
=>2 cos2 theta=a^2+1/(a^2) - hence first part is proved.

cubing both sides we get, (2 cos theta)^3=(a+1/a)^3
=>8cos^3 theta=a^3+1/(a^3)+3(a+1/a)
=>8cos^3 theta=a^3+1/(a^3)+3(2 cos theta) [from ques.]
=>8cos^3 theta-6 cos theta= a^3+1/(a^3)
=>2(4cos^3 theta - 3cos theta)=a^1/3 +1/(a^3)
=>2 cos3 theta =a^1/3 +1/(a^3) - hence second part is proved.
Cheers!!
Regards,
Vikas (B. Tech. 4th year
Thapar University)