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

1. 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!
2. 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
3. (2root5)(2root5) is the same as 2*2 multiplied by root5*root5 which is 4*5 which is 20
4. (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)
5. 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
6. (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!

Sorry about the quality of the picture. Let me know if you need help
7. 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
8. (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 . The question is actually .
9. (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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