Differences of powers

Algebra Level 4

What is the sum of all values of x x (in degrees) satisfying sin 12 x cos 12 x = 1 \sin^{12}x - \cos^{12}x = 1 , where 0 x 36 0 0^\circ \leq x \leq 360^\circ ?


The answer is 360.

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6 solutions

Ashish Bhatia
Aug 25, 2013

This is possible only when abs(sin x) = 1 => x = 90 or 270.

Tamoghna Banerjee
Aug 26, 2013

sin^12(x)−cos^12(x)=1 or, sin^12(x) = 1+cos^12(x) or, (sin^6(x))^2 = 1+(cos^6(x))^2 or, 1 - (cos^6(x))^2 = 1+(cos^6(x))^2 or, 2(cos^6(x))^2 = 0 or, (cos^6(x))^2 = 0 or, cos^6(x) = 0 or, cos(x) = 0 or, x = 90 and 270 or, Summation of all the values of x = 90+270 = 360 (in degrees.)

Gabriel Canovas
Aug 25, 2013

sin 12 x \sin^{12} x and cos 12 x \cos^{12} x both has a range of [0,1]

Therefore, in order for the equation to be satisfied, sin 12 x \sin^{12} x must be equal to 1 and cos 12 x \cos^{12} x must be equal to 0. This can only happen at x = 90 , 270 x=90,270

Russell Few
Aug 26, 2013

Note that s i n 12 x sin^{12}x and c o s 12 x cos^{12}x are nonnegative and are in the range [ 0 , 1 ] [0,1] . Since s i n 12 x c o s 12 x = 1 sin^{12}x-cos^{12}x=1 , s i n 12 x = 1 sin^{12}x=1 . Hence s i n x = ± 1 sin x=\pm 1 , so x = 9 0 x=90^\circ or 27 0 270^\circ . The sum of all possible values of x x is 90 + 270 = 360 90+270=\boxed{360} .

Moderator note:

From the way this solution is written, sin x = ± 1 \sin x = \pm 1 is merely a necessary condition, and not a sufficient one.

You still have to verify that 90 , 270 90, 270 are indeed solutions to the original equation.

note that if sin x=1, cos x=0, so sin^12 x - cos^12 x=1-0=1. Hence that is already a necessary and sufficient condition.

Russell FEW - 7 years, 9 months ago

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And of course, you have to do sin x = 1 \sin x = -1 ...

Calvin Lin Staff - 7 years, 9 months ago
Dhruv Sharma
Aug 31, 2013

Since the maximum value of s i n ( x ) sin(x) and c o s ( x ) cos(x) is 1 1 and since the twelfth power of either c o s ( x ) cos(x) or s i n ( x ) sin(x) is strictly positive, s i n 12 ( x ) = 1 \Rightarrow sin^{12}(x) = 1

x = 9 0 , 27 0 \Rightarrow x = 90^{\circ}, 270^{\circ} . Thus sum of all values of x is 36 0 360^{\circ}

Ovi N.
Aug 27, 2013

We add 2cos^12 x to both sides so that we have sin^12 x +cos^12 x=1+2cos^12 x . The RHS is > or = to 1, and the maximum of the LHS is 1 (because sin^2 x +cos^2 x =1, and both sin^12 and cos^12 are less than sin^2 and cos^2). Therefore, we solve the equation sin^12 x +cos^12 x=1, and this only has solutions when either sin^12 x or cos^12 x is equal to 1, because sin^12 x or cos^12 x<sin^2 x+cos^2 x for all other cases. sin^12 x=1 only at 90 and 270 degrees, and cos^12 x=1 at 0, 180, and 360 degrees. However, the cosine gives extraneous solutions, so we only take the ones from the sine, which are 90+270=360

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