Connect all nine dots with four straight lines in this imaginative puzzle.
A puzzle worksheet titled "Four lines" from MathSphere, featuring a 3x3 grid of dots and a cartoon mouse asking to connect all nine dots with four straight lines without lifting the pencil.
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Show Answer Key & Explanations
Step-by-step solution for: Free Math Puzzles | Thinkwell Homeschool
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Show Answer Key & Explanations
Step-by-step solution for: Free Math Puzzles | Thinkwell Homeschool
The problem is a classic puzzle: join all nine dots (arranged in a 3x3 grid) using only four straight lines, without lifting the pencil and without going over any line twice.
Let’s think step by step:
1. The dots are arranged like this:
• • •
• • •
• • •
2. If you try to stay within the “box” of the dots, you’ll find it impossible — that’s why the puzzle says it needs “imaginative thinking.”
3. The trick is to extend your lines *beyond* the edges of the dot grid. You’re not confined to the space between the dots.
4. Here’s how to do it:
- Start at the top-left dot.
- Draw a line diagonally down through the center dot to the bottom-right dot.
- Continue that same line beyond the bottom-right dot (outside the grid).
- From there, draw a line upward and to the left, passing through the middle-left dot and ending beyond the top-left dot.
- Now draw a horizontal line to the right, passing through the top row of dots (top-left, top-center, top-right), extending beyond the top-right dot.
- Finally, draw a diagonal line down through the center-right dot to the bottom-center dot — but wait, we’ve used 4 lines already? Let’s recount properly.
Actually, here’s the correct sequence:
Line 1: Start at top-left dot → go through top-center → top-right → continue past top-right.
Line 2: From where Line 1 ended, go diagonally down-left through center dot → bottom-left dot → continue past bottom-left.
Line 3: From there, go horizontally right through bottom-center → bottom-right → continue past bottom-right.
Line 4: From there, go diagonally up-left through center-right dot → top-center dot → and stop.
Wait — let me verify with a known solution:
Standard solution:
- Line 1: Top-left → top-center → top-right → extend beyond top-right.
- Line 2: From end of Line 1, go down-left through center → bottom-left → extend beyond bottom-left.
- Line 3: From end of Line 2, go right through bottom-center → bottom-right → extend beyond bottom-right.
- Line 4: From end of Line 3, go up-left through center-right → top-center → and stop.
But does this cover all 9 dots?
Let’s label them:
A B C
D E F
G H I
Line 1: A→B→C→beyond C
Line 2: beyond C → E → G → beyond G
Line 3: beyond G → H → I → beyond I
Line 4: beyond I → F → B → stop
Covered: A,B,C,E,G,H,I,F — missing D!
Oops — that doesn’t work.
Correct standard solution:
Start at top-left (A):
Line 1: A → D → G → extend below G
Line 2: from below G → go up-right through E → C → extend beyond C
Line 3: from beyond C → go left through B → A → but we can’t reuse A? Wait, no — we can pass through again? No, rule says “cannot go over any line twice” — meaning the path, not the dot. Dots can be revisited? Actually, no — the rule is “without going over any line twice”, so the line segments can’t overlap, but dots can be passed through multiple times? Actually, in this puzzle, you usually don’t revisit dots — but the rule doesn’t forbid it. However, the goal is to connect all dots, so each dot must be touched at least once.
Actually, the correct and proven solution:
Label dots:
1 2 3
4 5 6
7 8 9
Line 1: Start at 1 → 2 → 3 → extend beyond 3
Line 2: From beyond 3 → go down-left through 5 → 7 → extend beyond 7
Line 3: From beyond 7 → go right through 8 → 9 → extend beyond 9
Line 4: From beyond 9 → go up-left through 6 → 2 → stop? But 2 was already used.
Wait — better to use coordinates.
Set dots at (1,1), (1,2), (1,3)
(2,1), (2,2), (2,3)
(3,1), (3,2), (3,3)
Solution:
Line 1: Start at (1,1) → (2,2) → (3,3) → continue to (4,4)
Line 2: From (4,4) → (2,2) → (0,0) — but (0,0) is outside, and we pass through (2,2) again? Not ideal.
Actually, the most common correct solution:
Line 1: Top row: (1,1) → (1,2) → (1,3) → extend to (1,4)
Line 2: From (1,4) → (2,2) → (3,0) — wait, (3,0) is below bottom row.
Better:
Use this sequence:
- Start at top-left dot (position 1,1)
- Draw line down through middle-left (2,1) to bottom-left (3,1), then continue down to (4,1)
- From (4,1), draw line up-right through center (2,2) to top-right (1,3), then continue to (0,4)
- From (0,4), draw line down through middle-right (2,3) to bottom-right (3,3), then continue to (4,3)
- From (4,3), draw line up-left through bottom-middle (3,2) to top-middle (1,2)
Now check which dots are covered:
Line 1: (1,1), (2,1), (3,1)
Line 2: (2,2), (1,3) — and we passed through (1,3) already? No, first time. Also (0,4) is extra.
Actually, let's list all points touched:
After Line 1: dots (1,1), (2,1), (3,1)
After Line 2: adds (2,2), (1,3) — and we go to (0,4)
After Line 3: from (0,4) to (4,3): passes through (2,3), (3,3) — so adds (2,3), (3,3)
After Line 4: from (4,3) to (1,2): passes through (3,2), (1,2) — adds (3,2), (1,2)
Missing: (2,2) was added in line 2, (1,2) in line 4, (3,2) in line 4, (2,3) in line 3, etc.
List all 9:
(1,1) - L1
(1,2) - L4
(1,3) - L2
(2,1) - L1
(2,2) - L2
(2,3) - L3
(3,1) - L1
(3,2) - L4
(3,3) - L3
All covered! And we used 4 lines.
And we didn't lift pencil, didn't retrace any line segment.
Perfect.
So the answer is: It is possible by extending the lines beyond the grid of dots.
Final Answer: Yes, it is possible by drawing lines that extend beyond the outer dots, connecting all nine dots with four continuous straight lines without retracing.
Let’s think step by step:
1. The dots are arranged like this:
• • •
• • •
• • •
2. If you try to stay within the “box” of the dots, you’ll find it impossible — that’s why the puzzle says it needs “imaginative thinking.”
3. The trick is to extend your lines *beyond* the edges of the dot grid. You’re not confined to the space between the dots.
4. Here’s how to do it:
- Start at the top-left dot.
- Draw a line diagonally down through the center dot to the bottom-right dot.
- Continue that same line beyond the bottom-right dot (outside the grid).
- From there, draw a line upward and to the left, passing through the middle-left dot and ending beyond the top-left dot.
- Now draw a horizontal line to the right, passing through the top row of dots (top-left, top-center, top-right), extending beyond the top-right dot.
- Finally, draw a diagonal line down through the center-right dot to the bottom-center dot — but wait, we’ve used 4 lines already? Let’s recount properly.
Actually, here’s the correct sequence:
Line 1: Start at top-left dot → go through top-center → top-right → continue past top-right.
Line 2: From where Line 1 ended, go diagonally down-left through center dot → bottom-left dot → continue past bottom-left.
Line 3: From there, go horizontally right through bottom-center → bottom-right → continue past bottom-right.
Line 4: From there, go diagonally up-left through center-right dot → top-center dot → and stop.
Wait — let me verify with a known solution:
Standard solution:
- Line 1: Top-left → top-center → top-right → extend beyond top-right.
- Line 2: From end of Line 1, go down-left through center → bottom-left → extend beyond bottom-left.
- Line 3: From end of Line 2, go right through bottom-center → bottom-right → extend beyond bottom-right.
- Line 4: From end of Line 3, go up-left through center-right → top-center → and stop.
But does this cover all 9 dots?
Let’s label them:
A B C
D E F
G H I
Line 1: A→B→C→beyond C
Line 2: beyond C → E → G → beyond G
Line 3: beyond G → H → I → beyond I
Line 4: beyond I → F → B → stop
Covered: A,B,C,E,G,H,I,F — missing D!
Oops — that doesn’t work.
Correct standard solution:
Start at top-left (A):
Line 1: A → D → G → extend below G
Line 2: from below G → go up-right through E → C → extend beyond C
Line 3: from beyond C → go left through B → A → but we can’t reuse A? Wait, no — we can pass through again? No, rule says “cannot go over any line twice” — meaning the path, not the dot. Dots can be revisited? Actually, no — the rule is “without going over any line twice”, so the line segments can’t overlap, but dots can be passed through multiple times? Actually, in this puzzle, you usually don’t revisit dots — but the rule doesn’t forbid it. However, the goal is to connect all dots, so each dot must be touched at least once.
Actually, the correct and proven solution:
Label dots:
1 2 3
4 5 6
7 8 9
Line 1: Start at 1 → 2 → 3 → extend beyond 3
Line 2: From beyond 3 → go down-left through 5 → 7 → extend beyond 7
Line 3: From beyond 7 → go right through 8 → 9 → extend beyond 9
Line 4: From beyond 9 → go up-left through 6 → 2 → stop? But 2 was already used.
Wait — better to use coordinates.
Set dots at (1,1), (1,2), (1,3)
(2,1), (2,2), (2,3)
(3,1), (3,2), (3,3)
Solution:
Line 1: Start at (1,1) → (2,2) → (3,3) → continue to (4,4)
Line 2: From (4,4) → (2,2) → (0,0) — but (0,0) is outside, and we pass through (2,2) again? Not ideal.
Actually, the most common correct solution:
Line 1: Top row: (1,1) → (1,2) → (1,3) → extend to (1,4)
Line 2: From (1,4) → (2,2) → (3,0) — wait, (3,0) is below bottom row.
Better:
Use this sequence:
- Start at top-left dot (position 1,1)
- Draw line down through middle-left (2,1) to bottom-left (3,1), then continue down to (4,1)
- From (4,1), draw line up-right through center (2,2) to top-right (1,3), then continue to (0,4)
- From (0,4), draw line down through middle-right (2,3) to bottom-right (3,3), then continue to (4,3)
- From (4,3), draw line up-left through bottom-middle (3,2) to top-middle (1,2)
Now check which dots are covered:
Line 1: (1,1), (2,1), (3,1)
Line 2: (2,2), (1,3) — and we passed through (1,3) already? No, first time. Also (0,4) is extra.
Actually, let's list all points touched:
After Line 1: dots (1,1), (2,1), (3,1)
After Line 2: adds (2,2), (1,3) — and we go to (0,4)
After Line 3: from (0,4) to (4,3): passes through (2,3), (3,3) — so adds (2,3), (3,3)
After Line 4: from (4,3) to (1,2): passes through (3,2), (1,2) — adds (3,2), (1,2)
Missing: (2,2) was added in line 2, (1,2) in line 4, (3,2) in line 4, (2,3) in line 3, etc.
List all 9:
(1,1) - L1
(1,2) - L4
(1,3) - L2
(2,1) - L1
(2,2) - L2
(2,3) - L3
(3,1) - L1
(3,2) - L4
(3,3) - L3
All covered! And we used 4 lines.
And we didn't lift pencil, didn't retrace any line segment.
Perfect.
So the answer is: It is possible by extending the lines beyond the grid of dots.
Final Answer: Yes, it is possible by drawing lines that extend beyond the outer dots, connecting all nine dots with four continuous straight lines without retracing.
Parent Tip: Review the logic above to help your child master the concept of math puzzle worksheet high school.