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colormekaylee
08-01-2009, 04:29 PM
[3a - (2a-3)] [3a + (2a-3)]

I got 3a^2 - 4a^2 + 9

= -a^2 + 9

Is this right?

Subhotosh Khan
08-01-2009, 04:51 PM
[3a - (2a-3)] [3a + (2a-3)]

One way to do it:

= [3a - 2a + 3][ 3a + 2a - 3]

= [a + 3][5a - 3]

= 5a[sup:lg6720ko]2[/sup:lg6720ko] - 3a + 15a - 9

= 5a[sup:lg6720ko]2[/sup:lg6720ko] + 12a - 9

There is another way of doing it where you can use the formula x[sup:lg6720ko]2[/sup:lg6720ko] - y[sup:lg6720ko]2[/sup:lg6720ko] = (x-y)(x+y)

I got 3a^2 - 4a^2 + 9

= -a^2 + 9

Is this right?

DrMike
08-02-2009, 08:25 PM
[3a - (2a-3)] [3a + (2a-3)]

I got 3a^2 - 4a^2 + 9

= -a^2 + 9

Is this right?

No.

I think you tried to use (x-y)(x+y) = x[sup:3s022h10]2[/sup:3s022h10]-y[sup:3s022h10]2[/sup:3s022h10], but made some mistakes.

[3a - (2a-3)] [3a + (2a-3)] = (3a)[sup:3s022h10]2[/sup:3s022h10]-(2a-3)[sup:3s022h10]2[/sup:3s022h10], this is correct, but...
(3a)[sup:3s022h10]2[/sup:3s022h10] is not 3 a[sup:3s022h10]2[/sup:3s022h10], it is 9[sup:3s022h10]2[/sup:3s022h10].
Also, (2a-3)[sup:3s022h10]2[/sup:3s022h10] is not (2a)[sup:3s022h10]2[/sup:3s022h10] - 3[sup:3s022h10]2[/sup:3s022h10], it is (2a)[sup:3s022h10]2[/sup:3s022h10]-2.(2a).3+3[sup:3s022h10]2[/sup:3s022h10]=4a[sup:3s022h10]2[/sup:3s022h10]-12a+9.

Now, carry on from there, and let us know how you go...