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Start using Mumsnet Premium9^2 = 81
(193 Posts)I need a help from maths teachers.
My dc says her teacher taught the children the answer is 81. And when she has done the work on online maths programme, it's wrong obviously.
I have explained, it's 81, since it's really  (9^2). And to get answer as 81, the question need to be (9)^2, not 9^2. But being not a maths specialist, cannot explain why in a logical way , and the child blatantly believes the teacher, which is a good thing, but not really if the teacher is wrong.
How do you maths teachers explain this to your students?
(9)^2 is 81
Sorry I see you're agreeing. You wouldn't usually need to use brackets. The square of a negative is positive. Convention is that brackets are assumed.
9 squared it 81. Squaring gets ride of the negative
It depends on the question....
Is it substituting  9 into an expression?
Or is it just  9^2 as a standalone?
The convention is that exponents take precedence over negation. So if I see 3^2, with no brackets, I do the square first, taking care of the exponent, then I make it negative.
3^2=9
(3)^2=9
This is because in front of every  sign is an invisible zero. So the first sum above is "actually" 03^2, which hopefully is more obviously equal to 9 rather than 9.
StealthPolarBear, I know we normally don't need to use brackets. The question we got stuck was 9^2, so I used brackets to show my dc the difference in (9^2) and (9)^2. The rule of square of negative is positive. I know that. But it need to be (9)^2 or 9 x 9, instead of 9^2. to get 81. Seems to be very confusing when teacher says the answer for 9^2 is 81, but really, 9^2 = 81. And I can't explain well, the difference of having  in front of the number with powers.
It's a BODMAS (or PEDMAS or BIDMAS) thing. Squaring is an ORDER or EXPONENT, which means it is operated before the minus sign.
Oh fucking hell. All my carats made italics.
3 ^ 2 =  9. Because there is an invisible 0 in front of the negative number.
But thay is the convention. I don't understand why you're insisting it needs brackets.
A negative number squared is shown by x2 where the 2 is superscript.
Teachermaths, it's stand alone. So the answer is definitely 81.
9^2 = ?
ItsGoingTibiaK , that totally makes sense! Great. I think I got an answer how to explain. Thank you!
And thank you, Xiaoxiong. It's too complicated to understand the thing about 0 for me, but now I got it, that exponents take precedence over negation. I think I can explain properly now why. Thank you.
The answer is due to the order of operations. BODMAS brackets, orders, divisions, multiplications, additions, subtractions.
The exponent is an order, so the 9 is raised to its square of 81, and then either subtracted or multiplied by 1, whichever you use. 9 x 9 = 81 though.
This is why the quadratic equation is +/ at the beginning, because both the positive and negative answer are right (and why quadratic formulae make parabolas)
I see I've cross posted
It’s definitely 81 with no brackets. Brackets are needed to square the 9 and make the answer 81.
Thank you hiddenmnetter.
And thanks, everyone. I explained Bodmas/Pedmas and I think(hope) dc understood. Says so. And I asked her to ask the teacher if still not sure.
Funnily enough, I am leaning maths myself, and have encountered this question asked by someone else recently, and being an adult, I just accepted this as a fact, that was an only reason I knew 9^2 was 81. But don't have enough knowledge to explain it to someone else.
It helped me understand too. So, thank you.
Constant issue with my students making a mistake on this one. You are absolutely correct.
(9)^2 is in words  minus mine all to be squared.
9^2 is simply minus nine squared
The addition of brackets means that everything within the bracket is squared. Omit the brackets and you are only squaring the nine.
9^2 is definitely 81. Not negative. You can check by plugging into a calculator.
It is just 9 x 9 = 81
If it was (9^2), then it would be 81
I wouldn’t assume a calculator works on the correct order of operations  many don’t especially nonscientific ones. It is definitely 81. Maths grad and post grad plus maths job but also the correct answer of 81 is exactly what my primary and secondary school children are taught.
Charlie RTFT.
9 x 9 = (9)^2 not 9^2
CharlieWork, no you are wrong. It's not 9 x 9. It's  (9^2).=  1 x 81
Try using scientific calculator. By typing in 9^2, and typing in (9)^2 , you get different answers.
I have used a scientific calculator and have a maths degree! The square of a negative number is positive.
I have read the full thread, thank you
Calculators are programmed to work using the BODMAS method
I did Maths many many many years ago, and we never had the "presume there is a zero at the start of the expression". So whilst I appreciate that BODMAS says the ordinal comes first, tome the '' is not a minus sign (as in 09). I don't know the correct term but it's an indicator that the first number in the expression is 9 and not +9. It belongs to the 9, rather than standing alone. So to me, the equation is 9 squared which is +81.
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