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  • A shooting star crashes on Earth,

  • and a hideous blob emerges.

  • It creeps and leaps, it glides and slides.

  • It’s also unstoppable:

  • weapons, fire, extreme temperatures

  • no matter what you throw at it,

  • it just regrows and continues its rampage.

  • Its expansion is breathtaking;

  • it doubles in size every hour.

  • But there’s one opportunity:

  • after each hour, it goes to sleep,

  • forming itself into a flat triangle

  • and resting for a few minutes

  • before it begins eating and growing again.

  • Your only chance to save the planet

  • involves a satellite-mounted nano-fission ray that can cut through the blob.

  • When the blog is active

  • it heals itself within seconds.

  • However, when you break the sleeping blob into two triangles,

  • you make a critical discovery.

  • The acute triangle portion,

  • with all angles less than 90 degrees, is inert.

  • It neverwakes up.”

  • The obtuse triangle,

  • which has an angle greater than 90 degrees,

  • wakes up as usual and keeps growing.

  • Similar experiments show that all shapes other than acute triangles,

  • including right triangles, will also wake up.

  • For the next few minutes,

  • the blob is sleeping in its obtuse triangle form.

  • You can make clean, straight-line cuts

  • between any two points on or inside the triangle.

  • But youll only have time to make 7 cuts while the satellite is above you.

  • By the time it completes its orbit and returns,

  • the blob will have consumed the entire world,

  • if even a single portion that will wake up remains.

  • How can you cut the blob entirely into acute triangles

  • and stop it from destroying the planet?

  • Pause the video now to figure out for yourself

  • Answer in 3

  • Answer in 2

  • Answer in 1

  • While this seems doable at first,

  • there’s a hidden difficulty when it comes to avoiding obtuse and right angles.

  • Every time you make a cut that reaches an edge,

  • it either makes an acute and an obtuse angle, or two right angles.

  • That makes it seems like youre doomed to keep creating obtuse angles.

  • But as with so many of life’s problems,

  • we can look to pizza for inspiration.

  • Imagine squaring off the outside of a pizza,

  • so that instead of a circle, it’s an octagon.

  • When we cut it into slices,

  • each of the eight triangles is acute.

  • This works with larger polygons too.

  • Importantly, it also works for some polygons with fewer sides,

  • including heptagons, hexagons, and pentagons.

  • That’s good news,

  • because if you cut off the sharp corners of the blob triangle,

  • a pentagon is exactly what youll be left with.

  • And just like a pizza,

  • you can cut the blob pentagon into five acute triangles.

  • That’s 7 cuts, and it renders the blob completely inert.

  • Youve saved the day!

  • Now you just need to figure out what to do

  • with all of these giant, practically indestructible triangles.

A shooting star crashes on Earth,

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C1 TED-Ed blob acute triangle cut pentagon

Can you solve the unstoppable blob riddle? - Dan Finkel

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    林宜悉 posted on 2020/03/25
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