[Closed] If I cut a corner off a square, how do I work out what i'm left with?

It depends what you mean by "an opening of 1420mm"

The 1420 is the hypotenuse so the square of that, divided by 2 is the square of the length you need to deduct from 2860.
1856 is the number you asked for.

What angle is the opening relative to the sides?

Is there an assumption that the shortened sides are still equal in length?

If they are, the cutoff is a 45/45/90 triangle with a 1420mm hypotenuse. The ratios there are n:n:n×√2, so you can divide 1420 by the square root of 2 to find the length of the missing part of the side, then subtract that from your side length.

If the cut isn't even, you're going to need more info.

1856mm

This is just basic trig, isn't it, assuming your cut off corner has 2 equal lengths.

If a^2 + B^2 = c^2 and c^2 = 1,420mm

then a & b are 1,004mm and the remaining sides of the original square are 2,860-1,004 = 1,856mm

Not allowing for any thickness to your walls.
That's "external", if that makes sense.
Any thickness to your walls and the clear opening won't be 1420.
That's a different calc, depends exactly what you're trying to achieve.
Or cut the entrance at a 45 degree angle back into the square, maybe.

Damn, it's amazing what you forget when you don't use it.
Thx all

Not allowing for any thickness to your walls.
That’s “external”, if that makes sense.
Any thickness to your walls and the clear opening won’t be 1420.
That’s a different calc, depends exactly what you’re trying to achieve.

Good point - then the answer is probably just to buy a new bike

Has someone cut the carpet too short again?

6' 1 1/16"

If you cut each of the four corners off, you won't need to use Blutac in each corner to stick your poster to the wall.

Weigh it obvs.

Measure it.

Is no one going to ask: ‘Why? What are you making ?

A square has equal sides of 2860mm

Well I'd be a bit suspicious if they weren't equal.

This is a substantial flapjack.

Flatpjack is rectangular. Now millionaire shortbread...

How do you know the opening is 1420mm? Take whatever you used to measure that to measure the sides.