Finding Angles Of A Triangle With Sides

Three sides, three angles, and roughly 180 degrees of total interior space. That's where most people freeze up. That's the whole game with triangles, but figuring out each individual angle from just the side lengths? And honestly, I get it — geometry class taught a lot of us to memorize without really understanding.

Here's the good news: once you see the actual process, it's almost disappointingly simple. Consider this: the tool that makes it possible is called the Law of Cosines, and it's been quietly solving triangle problems since the 1800s. No calculus. No fancy software. Just one formula and a calculator (or even just a phone with a basic trig app).

Let me walk you through it the way I wish someone had explained it to me back in tenth grade.

What "Finding the Angles" Actually Means

You've got a triangle. On the flip side, you know the lengths of all three sides — let's say 7, 9, and 12 units, or whatever the problem gives you. You don't know a single angle. You want all three.

This is technically called solving a triangle by SSS (side-side-side), and it's one of three classic triangle-solving scenarios. Because of that, the other two are SAS (you know two sides and the angle between them) and ASA (you know two angles and a side). SSS is the trickiest of the three because you're starting from scratch with angles, and that's exactly why the Law of Cosines exists.

The Two Tools You'll Use

Law of Cosines. This is the heavy lifter. It connects one side of the triangle to the other two sides and the angle opposite to that side. The formula looks like this:

c² = a² + b² − 2ab·cos(C)*

Where C is the angle opposite side c, and a and b are the other two sides. You can rearrange it to solve for cos(C):

cos(C) = (a² + b² − c²) / (2ab)*

Then you take the inverse cosine (arccos or cos⁻¹ on most calculators) to get the actual angle.

Law of Sines. Once you know one angle, this gets you the other two way faster:

sin(A) / a = sin(B) / b = sin(C) / c*

So you solve the triangle with the Law of Cosines, then switch to the Law of Sines to finish the job.

Why This Skill Still Matters

You might be thinking — when am I ever going to use this? Fair question. The honest answer is that for most people, the actual computation rarely comes up outside of a classroom.

But here's the thing: the thinking* behind it shows up everywhere.

Surveyors use it to map land. In practice, game developers use it to figure out camera positioning and character movement. Architects lean on it to calculate roof angles and rafter lengths. Even your car's GPS uses triangulation principles that depend on these same relationships.

And beyond careers — there's a particular kind of satisfaction that comes from looking at three numbers and being able to figure out the full shape they describe. It's one of those small math wins that genuinely feels good.

How to Find All Three Angles, Step by Step

Let's go through a real example. Say the sides of your triangle are a = 5, b = 7, and c = 9. None of these are right angle clues (no Pythagorean triple), so we're definitely in Law of Cosines territory.

Step 1: Find the Largest Angle First

Always start with the biggest angle. The reason this matters: the largest angle is the one most prone to rounding error if you solve for it last. In real terms, it's opposite the longest side, which in this case is c = 9*. Get it first while your numbers are clean.

Plug into the rearranged Law of Cosines:

cos(C) = (5² + 7² − 9²) / (2 × 5 × 7)* cos(C) = (25 + 49 − 81) / 70* cos(C) = (−7) / 70* cos(C) = −0.1*

Now take the inverse cosine:

C = cos⁻¹(−0.1) ≈ 95.74°*

That negative cosine is a clue — it tells you the angle is obtuse, which makes sense for the biggest angle in a stretched-out triangle.

Step 2: Use the Law of Sines for the Next Angle

Now that you know C, you can find A:

sin(A) / 5 = sin(95.74°) / 9*

sin(95.74°) ≈ 0.9950*, so:

sin(A) = (5 × 0.9950) / 9 ≈ 0.5528*

A = sin⁻¹(0.5528) ≈ 33.56°*

Step 3: Get the Last Angle by Subtraction

You could run the Law of Sines one more time, but honestly? Just subtract. The three interior angles of any triangle always sum to 180°:

B = 180° − 95.74° − 33.56° ≈ 50.70°*

Done. Triangle fully solved.

Quick Sanity Check

Add them up: 95.Think about it: 74 + 33. Still, 56 + 50. Day to day, 70 = 180. Which means 00. Clean. The result is valid.

Common Mistakes That Throw People Off

Using the Wrong Side in the Formula

The Law of Cosines requires matching each angle with its opposite side. Worth adding: mix that up and your numbers will be off in confusing ways — not obviously wrong, just slightly* wrong, which is worse. Label your triangle clearly before you start. Every time.

For more on this topic, read our article on 11 out of 15 is what percentage or check out what is 20 off of $20.

Forgetting That the Inverse Cosine Can Give Two Answers

Calculators default to giving you the principal value (between 0° and 180°), which is exactly what you need for a triangle angle. But if you ever see strange results in a different context, remember that cos(60°) and cos(300°) are technically the same cosine value. For triangle problems, though, you're safe — just use the calculator's default.

Trusting the Law of Sines Too Early

So, the Law of Sines only works once you know at least one angle and its opposite side. If you try to use it with two sides and no known angle, you'll just spin your wheels. That's exactly the gap the Law of Cosines fills.

Rounding Too Early

Rounding your angle to a whole number before plugging it into the next calculation compounds the error. Keep at least four decimal places during the work, and round only at the end for your final answer.

Assuming a Triangle Exists

Not every set of three side lengths forms a real triangle. Sides of 2, 3, and 10? In practice, the Triangle Inequality Theorem says the sum of any two sides must be greater than the third. That's not a triangle. No angle-finding needed.

Practical Tips That Actually Help

Draw It First

Even if you're a visual thinker, sketch the triangle with the sides roughly proportional. It's not busywork — it helps you see which side is longest (and therefore which angle is biggest) before you touch a calculator.

Keep Everything in One Mode

If your calculator is in degree mode for one part of the problem, keep it in degree mode for the whole thing. Mixing radians and degrees is a classic way to get a result that's technically a number but practically nonsense.

Use the Law of Cosines Twice Instead of Mixing Methods

Some textbooks teach solving SSS triangles by using the Law of Cosines for all three angles. So that works fine and avoids any Law of Sines ambiguity with the supplementary-angle issue (the "ambiguous case" of SSA). For beginners, this is the safest route.

Double-Check With Heron's Formula for Area

If you want to verify your angles, calculate the area two different ways: once using Heron's formula (which only needs the sides) and once using ½·a·b·sin(C) (which uses your calculated angle). They should match. If they don't, something went wrong.

Memorize the Pattern, Not Just the Formula

The Law of Cosines always has the form: the side you don't have an angle for, squared, equals the sum of the other two sides squared, minus two times those sides times the cosine of the opposite angle.* Once you see the pattern, the formula stops feeling intimidating.

FAQ

Can you find angles of a triangle with only two sides?

No, you need a third piece of

information — either a third side or a third angle. Two sides alone can form infinitely many triangles by "swinging" the unknown side like a hinge.

What if I get a cosine value greater than 1 or less than -1?

That means a mistake was made somewhere upstream — either you used the wrong side, plugged in the wrong angle, or the sides don't actually form a valid triangle. Cosine outputs only ever fall in the range [-1, 1], so anything outside that range is a red flag to go back and recheck.

How accurate does my answer need to be?

For most classes, three to four decimal places during calculation and a final answer rounded to the nearest tenth or hundredth is standard. Match the precision your instructor expects, but don't carry more digits than necessary — it just makes arithmetic errors more likely.

Is the Law of Cosines ever wrong?

The formula itself is mathematically correct for any triangle. The only way it produces a "wrong" answer is through user error: misidentifying which side is opposite which angle, calculator mode mistakes, or arithmetic slips.

Can I use the Law of Cosines for right triangles?

Yes, but it's overkill. For a right triangle, the Pythagorean theorem is faster and simpler. The Law of Cosines actually reduces to the Pythagorean theorem when the angle is 90°, since cos(90°) = 0, which is a nice sanity check that you've written the formula correctly.

Conclusion

Mastering the Law of Cosines comes down to three things: knowing when to reach for it, setting up the equation carefully, and avoiding the small mistakes that can throw off an entire calculation. In real terms, use it whenever you have SAS (two sides and the included angle) or SSS (all three sides) — that's its sweet spot. For everything else, the Law of Sines or basic right-triangle trigonometry will serve you better. Which means keep your calculator in one mode, hold off on rounding until the end, and don't trust an answer that gives you an impossible cosine value. With a little practice, the formula becomes less of a thing to memorize and more of a reliable tool you reach for automatically. The triangles you encounter — whether in a geometry class, a physics problem, or a real-world measurement situation — all bow to the same rules, and now you've got the rules down.