Algebra

[DAnDan]

It's -20, no doubt about it.

 

[speedyj]

Come on people this isn't hard at all!

I'll even put it into words!

2 cubed is 8

2 times -4 is -8

-3 squared is 9, and 9 x 4 is 36

8 take away negative 8 is 16. 16 take away 36 is -20

That was simples.

I agree but the website shows different.

 

[DAnDan]

If the website shows different, then the website is wrong.

 

[Lydzor]

Its not my expression, I don't understand where that comes from either, it is on the website that my daughter was pointed to.

It looks wrong to me also.

For some reason for the 2b part of the equation it's done it twice:

Right:

b = -4

2 x b = -8

Instead, the website has done 2 x -4 first then also multiplied it again by b (-8) again for some reason.

 

[TheCenturion]

a=2, b=-4, c=-3

a³ - 2b - 4c²

As before, start with the "Not thinking" line:
2³ -8x(-4) -4x(-3)²

now sort out the numbers:
8 not 6!!!! It's 2x2x2 = 4x2 = 8
+ 32 minus times a minus is a plus
- 36 do it carefully, (-3)² = +9, -4x9 = -36
= 4

This is the explanation on a website my daughter has been pointed to by her teacher.

Where does the +32 come from?

it's -2 x 4, not -8 x -4

 

[speedyj]

Where does the +32 come from?

it's -2 x 4, not -8 x -4

Exactly!

My original post was to confirm that the website is wrong, I consider myself pretty able when it come to mathematics but was surprised that a website run by a maths teacher could be wrong. I will let my daughter ask the question of that teacher on Moday when she returns to school.

 

[TheCenturion]

The website is very wrong

might just have been a lil error they need to address, and not a gaping hole in the knowledge of the maths teacher

 

[Lightnix]

I think the teacher probably just thought 'what's 2x4... okay 8' and the substituted it in wrongly when they should have written that for the next step. Easy mistake to make but they should probably check for that before telling children it's right.