`sinx + cosx = 1`

`<=> sqrt2 sin(x + (\pi)/4) = 1`

`<=> sin(x + (\pi)/4) = 1/(sqrt2)`

`<=> sin(x + (\pi)/4) = sin((\pi)/4)`

`<=> x + (\pi)/4 = ((\pi)/4 + k2\pi` hoặc `x + (\pi)/4 = \pi - (\pi)/4 + k2\pi (k \in ZZ)`

`<=> x = k2\pi` hoặc `x = (\pi)/2 + k2\pi (k \in ZZ)`

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