Help with solving (x^3 - 3x + 5)(2x - 7) please?

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kitsae

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(x[sup:7kegomjh]3[/sup:7kegomjh]-3x+5)(2x-7)

This is how I attempted to solve it:

(1) I multiplied x[sup:7kegomjh]3[/sup:7kegomjh] by 2x and then by -7 to obtain 2x[sup:7kegomjh]3[/sup:7kegomjh] and -7x[sup:7kegomjh]3[/sup:7kegomjh] (I believe this is where I erred, please tell me why)

(2) I multiplied -3x by 2x and then by to obtain -6x[sup:7kegomjh]2[/sup:7kegomjh] and 21x

(3) I multiplied 5 by 2x and then by -7 to obtain 10x and -35

(4) I combined the numbers to obtain: -5x[sup:7kegomjh]3[/sup:7kegomjh]-6x[sup:7kegomjh]2[/sup:7kegomjh]+31x-35

The answer in the book is: 2x[sup:7kegomjh]3[/sup:7kegomjh]-13x[sup:7kegomjh]2[/sup:7kegomjh]+31x-35
 
Re: Help with this problem please?

kitsae said:
(x[sup:ng04g2m9]3[/sup:ng04g2m9]-3x+5)(2x-7) <<< are you sure this is 3

This is how I attempted to solve it:

(1) I multiplied x[sup:ng04g2m9]3[/sup:ng04g2m9] by 2x and then by -7 to obtain 2x[sup:ng04g2m9]4[/sup:ng04g2m9] and -7x[sup:ng04g2m9]3[/sup:ng04g2m9] (I believe this is where I erred, please tell me why)

(2) I multiplied -3x by 2x and then by to obtain -6x[sup:ng04g2m9]2[/sup:ng04g2m9] and 21x

(3) I multiplied 5 by 2x and then by -7 to obtain 10x and -35

(4) I combined the numbers to obtain: -5x[sup:ng04g2m9]3[/sup:ng04g2m9]-6x[sup:ng04g2m9]2[/sup:ng04g2m9]=31x-35

The answer in the book is: 2x[sup:ng04g2m9]3[/sup:ng04g2m9]-13x[sup:ng04g2m9]2[/sup:ng04g2m9]=31x-35

However if posted the problem correctly, the answer should be:

(x[sup:ng04g2m9]3[/sup:ng04g2m9]-3x+5)(2x-7)

= 2x[sup:ng04g2m9]4[/sup:ng04g2m9] - 7x[sup:ng04g2m9]3[/sup:ng04g2m9] - 6x[sup:ng04g2m9]2[/sup:ng04g2m9] + 31x - 35
Click to expand...

You can edit your own post - that is preferable as opposed to "explaining" it.
 
Re: Help with this problem please?

Hi, thanks, I edited my post. But I am not sure I understand your explanation of my problem.
 
Re: Help with this problem please?

Oh, ok, thanks. I guess the book is wrong.
 
Re: Help with this problem please?

Book answer would be correct if your problem was:

(x[sup:19e699fe]2[/sup:19e699fe]-3x+5)(2x-7)
 
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