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# S1 Maths - Not getting answer I need

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1. Hi. I'm currently doing an Edexcel S1 Maths past paper (Gold Tier 1) and on Question 4c, I know what I'm supposed to be doing, but I don't get the answer I want.

I got the mean to be 23.5, just as the mark scheme says, but my standard deviation keeps going wrong because my sigma fx^2 always turns out to be 354806.25 for some reason, rather than 37378.25.
I even put all the values in a list in my graphical calculator, with sigma fx^2 returning as 354806.25. And when manually calculating the sum of all these numbers squared I get this number. What's more confusing is that sigma fx is coming up as 1316.5 which is right, but strangely not sigma fx^2.
Can anybody help me identify where I'm going wrong? Thanks!
2. Ok - sum of fx^2, that is 6 * 5.5^2, + 15 * 15.5^2... etc

Try that. I feel like you've done 6^2 * 5.5, 15^2 * 15.5, instead of timesing the frequency by the MIDPOINT squared.

OR you've squared your sigma fx? Sigma fx^2 does NOT = (sigma fx)^2

I think it's the first mistake though.

Ah... I think you've done (6*5.5)^2 + (15*15.5)^2... etc? Right? You can't do that.

What you've done is sum of (fx)^2 which is sum of f^2 x^2, not the sum of f x^2
3. Jack, you're a lifesaver! Yeah, I did that. Not been paying enough attention to where the square symbol was.
I got it now, thanks! ^^
4. Any time!

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