urgent maths question! please

im sorry I cant stop panicking, I have this question on bounds
jon is drawing a quadrilateral
length of each side is 5.2cm to 1dp
complete the error interval for the length of one side

I thought you had to half 1 to get 0.5 and add and subtract from 5.2
but the markscheme uses 0.05 as their answer is 5.15 and 5.25

why is it 0.05

Reply 1

Sishmasquash

Original post by usernamenew

Ok so always go one decimal place further than the rounded length.
If 560.07 go 560.075 or 560.065
I just remember to go the last unit and add or subtract the '5' value from that

Hope this helps

Reply 2

usernamenew

Original post by Sishmasquash

Ok so always go one decimal place further than the rounded length.
If 560.07 go 560.075 or 560.065
I just remember to go the last unit and add or subtract the '5' value from that

Hope this helps

ok thank you, but that isnt the case all the time right? because most times when i just half 1dp, i use that and get it right.

Reply 3

Sishmasquash

Original post by usernamenew

ok thank you, but that isnt the case all the time right? because most times when i just half 1dp, i use that and get it right.

You half the dp of how many are present plus one present. So if it goes to one decimal place then then you must half 0.01 (2 dp)

If you get given a whole number like 5 the bounds are (0dp present +1) all divided by 2 = 0.5
so Ub 5 = 5.5 Lb 5 = 4.5

It's a weird one to explain

Reply 4

usernamenew

Original post by Sishmasquash

haha thank you, also when its significant figures doesnt it get more complicated?
how do you know for example
80 000 to 1sf
7400 to 2sf

is there a way of finding what to add and subtract when its significant figures

Reply 5

Sishmasquash

Original post by usernamenew

K, these are a bit different. If you have a large number (bigger than 1) count how many significant figures from the largest number right to left then round it up if the nextunit is 5 or bigger

3452 to 1sf is 3000
495783 to 3sf is 496000

For numbers between 0 and 1 (I highly doubt we would get asked negative numbers in this exam) go to the first unit that isn't 0 working from right to left then count however many sf from that number but you must include 0 once you start counting

0.00545 to 2sf is 0.0055 (the next value on from 2sf rounds it up because it is 5 or bigger)

0.0430404 to 5sf is 0.043040 <-- including the zero in the number and at the end

Hope this helps and best of luch today!

Reply 6

Towcestermaths

Original post by usernamenew

Its just the same as with decimal places. For the upper limit you go to the next column after the last relevant digit and add a 5.
If the number is 80,000 to 1sf then the upper bound will be 85,000 and the lower bound will be 75,000.
If the number is 7,400 to 2sf then the upper bound will be 7,450 and the lower bound 7,350

Reply 7

usernamenew

Original post by Sishmasquash

thank you so much but sorrry i didnt write clearly and specificly i was referring to bounds involving significant figures so when they ask you to round to the nearest signifcant figure instead of 1dp. thats when i get confused because when they ask to 1sf, you dont just half one all the time and add and subtract that from the number.