# Problem of the week #54 - April 8th, 2013

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#### Jameson

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Lots of participants this week!

Congratulations to the following members for their correct solutions:

1) MarkFL
2) Sudharaka
3) anemone
5) Petrus
7) agentmulder
8) Reckoner

Solution (from Reckoner):
We will make use of the identity $\sec^2 x - \tan^2 x = 1.$ This identity follows from the familiar Pythagorean identity, $\sin^2 x + \cos^2 x = 1,$ if you divide both sides by $\cos^2 x$ and rearrange the terms.

By factoring the original left-hand side, we have
\begin{align*}
\sec^4 x - \tan^4 x &= \left(\sec^2 x + \tan^2 x\right)\left(\sec^2 x - \tan^2 x\right)\\
&= \left(\sec^2 x + \tan^2 x\right)(1)\\
&= \left(\sec^2 x + \tan^2 x\right).
\end{align*}

\begin{align*} \text{L.H.S.}&= \sec^4x-\tan^4x\\ &=\left[\sec^2x\right]^2-\tan^4x\\ &=\left[1+\tan^2x\right]^2-\tan^4x\\ &=1+2\tan^2x+\tan^4x-\tan^4x\\ &=1+2\tan^2x\\ &=(\sec^2x-\tan^2x)+2\tan^2x\\ &=\sec^2x+\tan^2x &=\text{R.H.S.} \end{align*}