Verify if the equations sinx+cosx=0 and sinx-cosx=0 have the same solutions in the set [0,2pi)

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giorgiana1976 | College Teacher | (Level 3) Valedictorian

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Both equations are homogenous equations and they may be solved using tangent function.

We'll start with the 1st equation:


We'll divide by cos x:

sin x*/cos x + 1 = 0

tan x + 1 = 0

tan x = -1

x = arctan (-1)

The value of the tangent function is negative in the 2nd and the 4th quadrants.

x = - arctan 1

x = pi - pi/4

x = 3pi/4 (2nd quadrant)

x = 2pi - pi/4

x = 7pi/4 (4th quadrant)

Now, we'll solve the equation sinx-cosx=0.

tan x - 1 = 0

tan x = 1

The value of the tangent function is positive in the 1st and the 3rd quadrants.

x = pi/4 (1st quadrant)

x = pi + pi/4

x = 5pi/4 (3rd quadrant)

As we can notice, the x values, that represents the solutions of the given equations, are not the same for both: the 1st equation allows the set {3pi/4 ; 7pi/4} and the 2nd equation allows the set {pi/4 ; 5pi/4}.

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