Triangle classification worksheet for identifying types of triangles by sides and angles.

Let’s go triangle by triangle. We need to label each one with two things:

1. By sides: scalene (all sides different), isosceles (two sides same), or equilateral (all three sides same).
2. By angles: acute (all angles < 90°), obtuse (one angle > 90°), or right (one angle = 90°).

We’ll look at each triangle carefully and make our best guess based on how it looks — since we don’t have measurements, we use visual clues.

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Row 1:

- Triangle 1 (left): Looks like all sides are different → scalene. One angle looks bigger than 90° → obtuse.
→ *scalene, obtuse*

- Triangle 2 (middle): Very skinny, all sides look different → scalene. The top angle looks very small, but the bottom-left angle looks big — probably over 90°? Wait — actually, looking again, the longest side is opposite the biggest angle. Hmm… maybe it’s acute? No — let’s think: if one angle is clearly wider than a square corner, it’s obtuse. This one has an angle that looks sharper than 90°? Actually, no — wait, this triangle is tall and thin. The angle at the bottom might be close to 90°, but not quite. Let me check again — actually, in most worksheets like this, if it doesn’t look obviously right or obtuse, and all angles seem less than 90°, it’s acute. But this one — the left angle looks like it could be over 90°? I’m going to say obtuse because the base is short and the sides slope inward sharply — so the bottom angle is wide. And sides are all different → scalene.
→ *scalene, obtuse*

Wait — let me double-check. Maybe I should count grid squares? There are none. So rely on shape.

Actually, let’s try a better approach: for angle type, imagine placing a square corner (like the corner of a piece of paper) into each angle. If it fits exactly → right. If the angle is smaller → acute. If the angle is bigger → obtuse.

Triangle 2: The bottom angle — if you put a square there, the triangle’s angle would stick out beyond the square → so it’s obtuse. Yes.

- Triangle 3 (right): All sides look equal? Not quite — two sides look similar, third is a bit shorter? Or maybe all different? Actually, looks like all sides are different → scalene. All angles look less than 90° → acute.
→ *scalene, acute*

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Row 2:

- Triangle 4 (left): Two sides look about the same length → isosceles. All angles look less than 90° → acute.
→ *isosceles, acute*

- Triangle 5 (middle): Long and flat — one angle is clearly very wide → obtuse. Sides all different → scalene.
→ *scalene, obtuse*

- Triangle 6 (right): Looks like two sides are equal → isosceles. All angles look sharp → acute.
→ *isosceles, acute*

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Row 3:

- Triangle 7 (left): Two sides look equal → isosceles. All angles look less than 90° → acute.
→ *isosceles, acute*

- Triangle 8 (middle): Skinny, all sides different → scalene. One angle looks like it’s over 90°? The bottom angle — yes, looks wide → obtuse.
→ *scalene, obtuse*

- Triangle 9 (right): Very flat — one angle is clearly over 90° → obtuse. Sides all different → scalene.
→ *scalene, obtuse*

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Row 4:

- Triangle 10 (left): One angle looks like it’s over 90° → obtuse. Sides all different → scalene.
→ *scalene, obtuse*

- Triangle 11 (middle): Looks like it might have a right angle? The bottom-left angle — if you imagine a square corner, it fits! → right. Sides: two look equal? Left and bottom — maybe. Actually, looks like legs are equal → isosceles right triangle.
→ *isosceles, right*

- Triangle 12 (right): All sides look equal? Possibly → equilateral. In equilateral triangles, all angles are 60° → always acute.
→ *equilateral, acute*

Wait — is it really equilateral? It looks close, but maybe not perfect. In many worksheets, if it looks nearly equilateral, they expect “equilateral”. Also, all angles look same and less than 90° → definitely acute. For sides, if all appear equal, call it equilateral.

But let’s be careful — sometimes they trick you. Looking again: the triangle on bottom right — all sides do look approximately equal. So yes → equilateral, acute

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Now let’s list them in order (left to right, top to bottom):

1. scalene, obtuse
2. scalene, obtuse
3. scalene, acute
4. isosceles, acute
5. scalene, obtuse
6. isosceles, acute
7. isosceles, acute
8. scalene, obtuse
9. scalene, obtuse
10. scalene, obtuse
11. isosceles, right
12. equilateral, acute

Wait — let me recheck #2. Is it really obtuse? Let me visualize: it’s a tall, narrow triangle pointing up. The base is short. The two base angles are small, and the top angle is also small? That can’t be — sum must be 180°. If it’s tall and narrow, the top angle is small, and the two bottom angles are larger. Are either of them over 90°? Probably not — maybe I was wrong earlier.

Actually, in a tall skinny triangle like that, often all angles are acute. For example, think of a slice of pizza cut very thinly — all angles are less than 90°.

So maybe #2 is acute, not obtuse.

Let me correct:

Triangle 2: Tall, narrow, apex at top. Base angles are large but likely still under 90°. Top angle is small. So all angles < 90° → acute. Sides all different → scalene.

So correction: #2 → *scalene, acute*

Similarly, check #8: middle of row 3 — it’s leaning, but is the bottom angle really obtuse? Let’s see: it’s pointing down-left. The angle at the bottom — if you draw a horizontal line, the side goes up-right and down-left — the angle between them might be over 90°. I think it is obtuse. Okay, keep as obtuse.

#9: very flat, almost a straight line — definitely one angle over 90° → obtuse.

#10: similar — one angle clearly wide → obtuse.

#11: looks like a right triangle — yes, and two legs equal → isosceles right.

#12: equilateral — yes.

Also, #3: I said scalene acute — looks good.

#6: isosceles acute — yes.

#7: isosceles acute — yes.

#4: isosceles acute — yes.

#5: scalene obtuse — yes.

#1: scalene obtuse — yes.

So revised list:

1. scalene, obtuse
2. scalene, acute ← corrected
3. scalene, acute
4. isosceles, acute
5. scalene, obtuse
6. isosceles, acute
7. isosceles, acute
8. scalene, obtuse
9. scalene, obtuse
10. scalene, obtuse
11. isosceles, right
12. equilateral, acute

I think that’s accurate based on visual inspection.

Final Answer:
1. scalene, obtuse
2. scalene, acute
3. scalene, acute
4. isosceles, acute
5. scalene, obtuse
6. isosceles, acute
7. isosceles, acute
8. scalene, obtuse
9. scalene, obtuse
10. scalene, obtuse
11. isosceles, right
12. equilateral, acute