Solve cos(x) = √(2)/2
Trigonometric equation, worked out line by line the way a teacher would write it.
Answer
| General solution | x = π/4 + 2π·n; x = 7π/4 + 2π·n (n any integer) |
Step-by-step solution
6 steps-
1 Given\cos{\left(x \right)} = \frac{\sqrt{2}}{2}
Solve for x.
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2 Reference angle: the acute angle α with cos α = √2/2
The reference angle α is the acute angle (between 0 and 90°) whose sine, cosine or tangent has this value, ignoring the sign. The other angles with the same value are built from it in the next step.
\alpha = \arccos\left(\frac{\sqrt{2}}{2}\right) = \frac{\pi}{4} \approx 0.7853981256 -
3 Cosine is positive in quadrants I and IV: u = α or u = 2π − α
In one full turn, sine and cosine reach each value between −1 and 1 twice. The signs follow the quadrants: everything is positive in quadrant I, only sine in II, only tangent in III and only cosine in IV.
x = \frac{\pi}{4}\quad \text{or} \quad x = \frac{7 \pi}{4} -
4 General solution (n is any integer)
Sine and cosine repeat every full turn (360° or 2π) and tangent every half turn, so adding any whole number n of periods gives another solution. n can be any integer, positive, negative or 0.
x = \frac{\pi}{4} + 2 \pi n\quad \text{or} \quad x = \frac{7 \pi}{4} + 2 \pi n,\quad n \in \mathbb{Z} -
5 The solutions in one turn, 0 ≤ x < 2πx = \frac{\pi}{4},\; \frac{7 \pi}{4}
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6 Check\begin{aligned}x = \frac{\pi}{4}:\quad \frac{\sqrt{2}}{2} = \frac{\sqrt{2}}{2}\quad\checkmark\\ x = \frac{7 \pi}{4}:\quad \frac{\sqrt{2}}{2} = \frac{\sqrt{2}}{2}\quad\checkmark\end{aligned}
Substituting each solution back makes both sides equal.
Check by substitution
| x | Left side | Right side | |
|---|---|---|---|
| pi/4 | √2/2 | √2/2 | ✓ |
| 7pi/4 | √2/2 | √2/2 | ✓ |
Change a number or try your own problem: the solver works it out the same way, with a graph and hint mode.