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Solve the equation. Sinx = 1 + Cos^2x.

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frontman0 | Student, Undergraduate | (Level 1) Honors

Posted May 29, 2009 at 10:30 PM via web

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Solve the equation.

Sinx = 1 + Cos^2x.

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neela | High School Teacher | (Level 3) Valedictorian

Posted May 29, 2009 at 11:33 PM (Answer #1)

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To solve the equation sinx=1+cos^2x, we use the trigonometrical identiy sin^2+cos^2=1

From the above identity, cos^2x =1-sin^2x.Replacing this fact in the given equation we get:

sinx=1+(1-sin^2x). Rearrange this as a quadratic in sinx we get,

sin^2x+sinx-2=0=>{sinx+(1/2)}^2-(1/2)^2-2 =0=>

sinx+1/2 =sqrt(2.25) =+1.5 or -1.5

sinx=1.5-0.5 =1 or sinx =-1.5-0.5=-2.0 is not feasible, as sinx is always obeying the inequality, -1<=sinx<=1.

Thus sinx =1 gives x=Pi/2 is the only practical solution  or x=Pi/2+2nPi is the general solution.

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tonys538 | TA , Undergraduate | (Level 1) Valedictorian

Posted December 29, 2014 at 6:22 AM (Answer #2)

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The equation sin x = 1 + cos^2x has to be solved.

sin x = 1 + cos^2x

Use the relation sin^2x + cos^2 x = 1 or cos^2x = 1 - sin^2x

The given equation can be written as:

sin x = 1 + 1 - sin^2x

sin^2x + sin x - 2 = 0

sin^2x + 2sinx - sin x - 2 = 0

sin x(sin x + 2) - 1(sin x + 2) = 0

(sin x - 1)(sin x + 2) = 0

sin x = 1 and sin x = 2

But the value of sinusoidal functions sin x and cosine x lies in the set [-1, 1]

Therefore the root sin x = 2 can be ignored.

sin x = 1 gives x = 90 degrees.

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