Answer: -4/3
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Explanation:
Let's square both sides and do a bit of algebra to get the following.
[tex]\sin(x) + \cos(x) = 1/5\\\\\left(\sin(x) + \cos(x)\right)^2 = \left(1/5\right)^2\\\\\sin^2(x) + 2\sin(x)\cos(x) + \cos^2(x) = 1/25\\\\\sin^2(x) + \cos^2(x) + 2\sin(x)\cos(x) = 1/25\\\\1 + 2\sin(x)\cos(x) = 1/25\\\\\sin(2x) = 1/25 - 1\\\\\sin(2x) = 1/25 - 25/25\\\\\sin(2x) = -24/25\\\\[/tex]
Now apply the pythagorean trig identity to determine cos(2x) based on this. You should find that cos(2x) = -7/25
This then means tan(2x) = sin(2x)/cos(2x) = 24/7.
From here, you'll use this trig identity
[tex]\tan(2x) = \frac{2\tan(x)}{1-\tan^2(x)}\\\\[/tex]
which is the same as solving
[tex]\tan(2x) = \frac{2w}{1-w^2}\\\\[/tex]
where w = tan(x)
Plug in tan(2x) = 24/7 and solve for w to get w = -4/3 or w = 3/4
So either tan(x) = -4/3 or tan(x) = 3/4.
If we were to numerically solve the original equation for x, then we'd get roughly x = 2.21; then notice how tan(2.21) = -1.345 approximately when your calculator is in radian mode.
Since tan(x) < 0 in this case, we go for tan(x) = -4/3
