What is the sum of all values of x (in degrees) satisfying sin 1 2 x − cos 1 2 x = 1 , where 0 ∘ ≤ x ≤ 3 6 0 ∘ ?
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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.)
sin 1 2 x and cos 1 2 x both has a range of [0,1]
Therefore, in order for the equation to be satisfied, sin 1 2 x must be equal to 1 and cos 1 2 x must be equal to 0. This can only happen at x = 9 0 , 2 7 0
Note that s i n 1 2 x and c o s 1 2 x are nonnegative and are in the range [ 0 , 1 ] . Since s i n 1 2 x − c o s 1 2 x = 1 , s i n 1 2 x = 1 . Hence s i n x = ± 1 , so x = 9 0 ∘ or 2 7 0 ∘ . The sum of all possible values of x is 9 0 + 2 7 0 = 3 6 0 .
From the way this solution is written, sin x = ± 1 is merely a necessary condition, and not a sufficient one.
You still have to verify that 9 0 , 2 7 0 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.
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And of course, you have to do sin x = − 1 ...
Since the maximum value of s i n ( x ) and c o s ( x ) is 1 and since the twelfth power of either c o s ( x ) or s i n ( x ) is strictly positive, ⇒ s i n 1 2 ( x ) = 1
⇒ x = 9 0 ∘ , 2 7 0 ∘ . Thus sum of all values of x is 3 6 0 ∘
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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This is possible only when abs(sin x) = 1 => x = 90 or 270.