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Prove the identity (sinx)^2 - (cosx)^2 = (sinx)^4 - (cosx)^4

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agneslund | Student, Undergraduate | (Level 1) Honors

Posted February 25, 2011 at 7:58 PM via web

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Prove the identity (sinx)^2 - (cosx)^2 = (sinx)^4 - (cosx)^4

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

Posted February 25, 2011 at 8:02 PM (Answer #1)

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For the beginning, we'll manipulate the right side, only.

[(sin x)^2]^2 - [(cos x)^2]^2 = [(sin x)^2 - (cos x)^2][(sin x)^2 + (cos x)^2]

But, (sin x)^2 + (cos x)^2 = 1 (Pythagorean identity)

[(sin x)^2]^2 - [(cos x)^2]^2 = [(sin x)^2 - (cos x)]^2

We notice that we've get the difference of squares from the left side:

[(sin x)^2 - (cos x)]^2 = [(sin x)^2 - (cos x)]^2

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mathsworkmusic | (Level 1) Educator

Posted March 8, 2015 at 5:57 PM (Answer #2)

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A simpler way to see what is going on in this equation is to redefine the essential terms. Specifically, let

(sinx)^2 = r

(cosx)^2 = s

The equation can then be written as

r - s = r^2 - s^2

We can then use the method of factorizing the difference of two squares, namely

r^2 - s^2 = (r - s)(r + s)

Note that the 'cross terms' rs and -rs cancel each other out.

Therefore we can now write the equation as

r - s = (r - s)(r + s)

From the (Pythagorean, or, unit circle) trigonometric identity  (sinx)^2 + (cosx)^2 = 1  we have that

r + s = 1

Our equation then can be written as

r - s = (r - s) x 1

that is

r - s = r - s

As this holds as true, we see that the original equation does indeed hold true.

NB If you plot (sinx)^2 + (cosx)^2 on a graph the identity (sinx)^2 + (cosx)^2 = 1 is apparent as the curves are perfectly symmetric in the line y = 1/2

(sinx)^2 - (cosx)^2 = (sinx)^4 - (cosx)^4


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gsenviro | College Teacher | (Level 1) Senior Educator

Posted March 10, 2015 at 2:23 AM (Answer #3)

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consider the identity, `x^2 -y^2 = (x-y)(x+y)`.

using the same on the right hand side of the equation, we get:

`(sinx)^4 - (cosx)^4 = [(sinx)^2 - (cosx)^2)].[(sinx)^2 + (cosx)^2)]`

Using the identity, `(sinx)^2 + (cosx)^2) = 1`, this equation simplifies to:

`(sinx)^4 - (cosx)^4 = [(sinx)^2 - (cosx)^2)].[(sinx)^2 + (cosx)^2)] = (sinx)^2 - (cosx)^2)`

which is the same as the left hand side of the equation. and hence proved.

In general, for this type of mathematical problems, its always ideal to start by one side of the equation (either LHS or RHS, i.e. left hand side or right hand side) and simplifying it. Use of identities, if applicable, helps in simplification process. and finally we get the two sides as equal to each other, i.e., the requisite answer.

Good luck.

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