Moaki M.

asked • 11/14/17

Proof:cot^2(A)+1/cos^2(A)-1=sec(2A)

Plz help me solve this sum, I will really appreciate it.

Michael J.

Work out the right side,
 
We have
 
1 / cos(2A) =
 
1 / (2cos2A - 1) =
 
 
The right side does not match the left side.  Therefore, it is not an identity.
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11/14/17

Kenneth S.

Student, please revise this carefully, in particular with respect to parenthesization.
Whenever you have a numerator consisting of multiple terms, parenthesize that separately.
Similarly for a denominator consisting of more than one term.
Otherwise what you write is ambiguous because here, unlike on a sheet of paper, you do not have the luxury of a long division symbol that also acts as a grouping symbol.
Unless it's very clear what you intend, it's not worthwhile trying to prove something weird.
(This is a PEMDAS issue.)
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11/14/17

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