Prove that: cos²x + cos^2(x+π/3) + cos^2 (x-π/3) = 3/2
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Answered by
1
Answer:
LHS=cos
2
x+cos
2
(x+
3
π
)+cos
2
(x−
3
π
)
Weknowthat,cos2x=2cos
2
x−1
∴cos
2
x=
2
cos2x+1
∴cos
2
(x+
3
π
)=
2
cos2(x+
3
π
)+1
∴cos
2
(x−
3
π
)=
2
cos2(x−
3
π
)+1
∴LHS=1+
2
cos2x
+
2
1+cos(2x+
3
2π
)
+
2
1+cos(2x−
3
2π
)
=
2
1
[1+1+1+cos2x+cos(2x+
3
2π
)+cos(2x−
3
2π
)]
=
2
1
[3+cos2x+2cos
⎝
⎜
⎜
⎜
⎛
2
2x+2x+
3
2π
−
3
2π
⎠
⎟
⎟
⎟
⎞
].
⎝
⎜
⎜
⎜
⎛
cos
⎝
⎜
⎜
⎜
⎛
2
2x−2x+
3
2π
+
3
2π
⎠
⎟
⎟
⎟
⎞
⎠
⎟
⎟
⎟
⎞
=
2
1
[3+cos2x+2cos2x.cos
3
2π
]
=
2
1
[3+cos2x+2cos2x(−cos
3
π
)]
=
2
1
[3+cos2x−cosx]
=
2
3
=RHS
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