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Why doesn't this (2root5)^2 simplify properly when I expand the brackets? watch

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    When I do it as (2 root 5)(2 root 5) because the bracket is squared, I end up with an answer of 9+ 4 root 5, which is wrong.

    The correct answer is 20, which you get by doing the 2^2 which gives 4, and the (root5)^2 which gives 5, so you do 5*4=20 and that is the correct answer.

    Why doesn't my method of expanding out the brackets work? When I expand it and multiply out, I get 4 + (2 root 5) + (2 root 5) +5 = 9 + (4 root 5).

    Why doesnt that work??

    Thanks!
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    It's multiplication, not addition. If it was (2+rt5)^2, your answer would be right. In this case, you're multiplying, so (2rt5)^2 is the same as 4*5=20
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    (2root5)(2root5) is the same as 2*2 multiplied by root5*root5 which is 4*5 which is 20
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    (Original post by blobbybill)
    When I do it as (2 root 5)(2 root 5) because the bracket is squared, I end up with an answer of 9+ 4 root 5, which is wrong.

    The correct answer is 20, which you get by doing the 2^2 which gives 4, and the (root5)^2 which gives 5, so you do 5*4=20 and that is the correct answer.

    Why doesn't my method of expanding out the brackets work? When I expand it and multiply out, I get 4 + (2 root 5) + (2 root 5) +5 = 9 + (4 root 5).

    Why doesnt that work??

    Thanks!
    Because it is (2root5) * (2root5), not (2 + root5) * (2 + root5).

    Let's say root5 = x
    The question is asking for (2x) * (2x)
    But you're doing (2 + x) * (2 + x)
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    It's multiplication, so you don't use the ac+ad+bc+bd (or a^2 + 2ab + b^2) rule, you just expand to (2*2)*((5^.5)*(5^.5)) --> 20
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    (Original post by blobbybill)
    When I do it as (2 root 5)(2 root 5) because the bracket is squared, I end up with an answer of 9+ 4 root 5, which is wrong.

    The correct answer is 20, which you get by doing the 2^2 which gives 4, and the (root5)^2 which gives 5, so you do 5*4=20 and that is the correct answer.

    Why doesn't my method of expanding out the brackets work? When I expand it and multiply out, I get 4 + (2 root 5) + (2 root 5) +5 = 9 + (4 root 5).

    Why doesnt that work??

    Thanks!
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    You're squaring (2 + root5) instead of (2 x root5).

    (2 x root5)^2
    = (2 x root5)(2 x root5)

    because it's just all multiplication, you don't need the brackets:

    2 x root5 x 2 x root5

    multiplication can be done in any order so:

    2 x 2 x root5 x root5
    = 4 x 5
    = 20
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    (Original post by blobbybill)
    When I do it as (2 root 5)(2 root 5) because the bracket is squared, I end up with an answer of 9+ 4 root 5, which is wrong.

    The correct answer is 20, which you get by doing the 2^2 which gives 4, and the (root5)^2 which gives 5, so you do 5*4=20 and that is the correct answer.

    Why doesn't my method of expanding out the brackets work? When I expand it and multiply out, I get 4 + (2 root 5) + (2 root 5) +5 = 9 + (4 root 5).

    Why doesnt that work??

    Thanks!
    You've worked out the answer to (2+\sqrt{5})^2. The question is actually (2\sqrt{5})^2 = 2^2\sqrt{5}^2 = 4\times5 = 20.
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    (Original post by blobbybill)
    When I do it as (2 root 5)(2 root 5) because the bracket is squared, I end up with an answer of 9+ 4 root 5, which is wrong.

    The correct answer is 20, which you get by doing the 2^2 which gives 4, and the (root5)^2 which gives 5, so you do 5*4=20 and that is the correct answer.

    Why doesn't my method of expanding out the brackets work? When I expand it and multiply out, I get 4 + (2 root 5) + (2 root 5) +5 = 9 + (4 root 5).

    Why doesnt that work??

    Thanks!
    The answer would be 20. Look at other user's working out.
 
 
 
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