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Answer:
cos () =
Decimal value: −0.9238795325...
Step-by-step explanation:
Write,
cos = cos
= −cos
Now using half angle formula, cos(x) = 2 cos²() − 1, we can write cos
= 2
cos²() − 1
This means + 1 = 2 cos²(
)
2 cos²() =
cos²() =
cos() =
Simplify,
=
=
Hence,
cos () = −cos (
) =
Given:
The expression is
To find:
The error in Frankie's work.
Solution:
We need to simplify , so
and,
Now,
Therefore, there is error in Frankie's last step. In that step, he did not subtract the mixed fraction correctly.