P5 math
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PQ was worked out to be 12
So I took 42/12 x 2 = 7 (cos area of triangle is ½ L x H)
So take 16-7 = 9,
Hence ½ x 30x9 = 135
Area of rectangle = 480 – 135 = 345 shaded..
But the given answer : find area of ABQ + area of DCP then minus 42cm2 = 294cm2
Why?? I cannot understand y my method is wrong, I use the formula to find area of triangle to find ht, anything wrong??
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Hmmm....okay, there's something I dun get for the question.
You mean PO = OQ, right? I mean, there's two "Q" in the question.
EDIT: Okay, PO = OQ and PQ = QC. Ok. -
Okay, long time never did this kind of thing but it's actually quite easy.
From diagram, you know PQ = QC and BP = (1/2)QC
So let BP be 1 unit --> QC = 2 units = PQ
---> 5 units = 30 cm ---> 1 unit = 6 cm
Hence,
PC = PQ + QC = 24 cm
BQ = BP + PQ = 18 cm
Therefore,
Area ABQ = 1/2 * 16 * 18 = 144 cm^2
Area CDP = 1/2 * 16 * 24 = 192 cm^2
---> Area of shaded portion = 144 + 192 - 42 = 294 cm^2 which is your answer.
And as for the discrepancy in answer, actually if you draw out the rectangle, you realise that the area of the shaded portion isn't really 42 cm^2.
That is, the entire diagram has inconsistent dimensions. Your method is correct and the 294 is also "correct" but because the dimensions are inconsistent, you'll get different answers. -
Well, you can always double check mah...Originally posted by a_splash:
PQ was worked out to be 12
So I took 42/12 x 2 = 7 (cos area of triangle is ½ L x H)
So take 16-7 = 9,
Hence ½ x 30x9 = 135
Area of rectangle = 480 – 135 = 345 shaded..
But the given answer : find area of ABQ + area of DCP then minus 42cm2 = 294cm2
Why?? I cannot understand y my method is wrong, I use the formula to find area of triangle to find ht, anything wrong??
area of the whole rectangle is 30x16=480.
PQ=QC, BP=1/2QC
SO THE LENGTH OF BC is broken into 5 parts right? like PQ=2 PARTS=QC, BP=1 part.
So 30/5 is 6cm.
Find ABQ= 1/2 X 16 X 6(3) = 144.
Now, Find DCP = 1/2 X 16 X 6(4) = 192.
So, DCP-POQ=192-42= 150.
So shaded area is 150+144=294.
Thank you, xie xie.
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Coolmyth beat me by 4 min!!!!
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We can redo the dimensions as follows:
We know BP = 6 cm, PQ = 12 cm, QC = 12 cm
Then, by drawing the rectangle ABCD, what I get actually was:
PO = 8.2 cm and OQ = 6.8 cm
Let X be the point where O meets PQ at 90 degrees. And thus OX = 4.6 cm
--> Area POQ = 1/2 * 12 * 4.6 = 27.6 cm^ 2 which is not 42.
So, the dimensions are inconsistent. -
hi coolmyth and nightzip...
thank you!! Xie Xie
u r rite coolmyth,...
today i literally draw out the pix in actual measurement and i get the ht of POQ to be 4.6, so the area cannot be 42cm2
Yoz so the book has error....headache ley like that......
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Well, even PSLE can have this kind of thing happen already.Originally posted by a_splash:hi coolmyth and nightzip...
thank you!! Xie Xie
u r rite coolmyth,...
today i literally draw out the pix in actual measurement and i get the ht of POQ to be 4.6, so the area cannot be 42cm2
Yoz so the book has error....headache ley like that......
The problem with people setting this type of question is that they never checked the diagrams are mathematically consistent. -
i think they have some changes in this types of questions that will tested for PSLE now (following that problem faced last year) i need to go and update myself first....Originally posted by CoolMyth:Well, even PSLE can have this kind of thing happen already.
The problem with people setting this type of question is that they never checked the diagrams are mathematically consistent.
i can understand now the stress those children went thru last yr....i was not willing to give up yesterday....trying this and that until i go crazy before i tot of drawing the actual measurement..... 
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Well, primary school maths doesn't really test the mathematical ability of the student.Originally posted by a_splash:i think they have some changes in this types of questions that will tested for PSLE now (following that problem faced last year) i need to go and update myself first....
i can understand now the stress those children went thru last yr....i was not willing to give up yesterday....trying this and that until i go crazy before i tot of drawing the actual measurement.....
Rather, it tests how unorthodox the student can think and how "out-of-the-box" the student can go. Especially now.
So, can understand the stress involved. Heck, even teachers are facing difficulty in setting "good" questions. -
and test your english standard too.....Originally posted by CoolMyth:Well, primary school maths doesn't really test the mathematical ability of the student.
Rather, it tests how unorthodox the student can think and how "out-of-the-box" the student can go. Especially now.
So, can understand the stress involved. Heck, even teachers are facing difficulty in setting "good" questions. ( if one is not too good in english can also die flat...haha) -
thanks once again coolmyth!!
i m logging out....
splash off!! -
coolmyth.....
help needed again......
how do u find the answer for 2 +4+6 ........+ 100??
thanks alot! -
I'll assume this is a primary school question. No usage of algebra and stuff.Originally posted by a_splash:coolmyth.....
help needed again......
how do u find the answer for 2 +4+6 ........+ 100??
thanks alot!
Through close observation, you can see that the first digit and the last digit always adds up to 102 (2 + 100)
Take those two out and you see the same thing (4 + 98 = 102)
So, you just need to find the number of terms in this series. Therefore,
Number of terms = 100 - 50 (100 terms from 1 to 100 minus away the odd numbers. Note that there are equal number of even and odd terms from 1 to 100) = 50
Hence, the answer is 102 x (50/2) = 2,550. (Remember you're "pairing" two numbers together to make 102)
In sec school, this is simply just summation from n=1 to 50 (2n) and we have a formula that says summation n terms = (n/2)(n+1). -
ok wait let me digest slowly......
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No prob. I'm just being long-winded.Originally posted by a_splash:ok wait let me digest slowly...... -
umm this one is very familiar.....wat do u call this equation or this method??
In sec school, this is simply just summation from n=1 to 50 (2n) and we have a formula that says summation n terms = (n/2)(n+1).[/b] -
your explanation is good for old ppl like me.....Originally posted by CoolMyth:No prob. I'm just being long-winded. -
ok understand le.....so its actually so simple?? woot.......thanks again!!
wish i hv tougher questions for u..... (kidding) -
The topic is under Summation Series, I think. Normally, when they introduce the summation symbol, this is the first thing they learned.Originally posted by a_splash:umm this one is very familiar.....wat do u call this equation or this method?? -
think of all those formulas i head big big....Originally posted by CoolMyth:The topic is under Summation Series, I think. Normally, when they introduce the summation symbol, this is the first thing they learned.
u specializing in maths??? peifu! -
The first time a person does this, he/she might have problems.Originally posted by a_splash:ok understand le.....so its actually so simple?? woot.......thanks again!!
wish i hv tougher questions for u..... (kidding)
But once you know how to handle this type of question, you can handle similar ones as well.
Example: 1 + 3 + 5 + ..... + 301, 2 + 6 + 10 + .... + 66 etc.
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ya this one seems like once the method known....very easy liao....Originally posted by CoolMyth:The first time a person does this, he/she might have problems.
But once you know how to handle this type of question, you can handle similar ones as well.
Example: 1 + 3 + 5 + ..... + 301, 2 + 6 + 10 + .... + 66 etc. -
Some formulas will always stay in the head once you use them long enough. Some of them I still have to refer to books.Originally posted by a_splash:think of all those formulas i head big big....
u specializing in maths??? peifu!
And ya, I major in maths. Though what I'm learning now is vastly different from what people think maths is like. -
so when u major in maths.....how many of the subjects u do each yr related to maths?Originally posted by CoolMyth:Some formulas will always stay in the head once you use them long enough. Some of them I still have to refer to books.
And ya, I major in maths. Though what I'm learning now is vastly different from what people think maths is like.
u r in Uni i guess...