Answer the question

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tauhid islam
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Answer the question

Unread post by tauhid islam » Tue Sep 27, 2016 10:53 am

if $a=2x-3$, $b=2x+5$ and $x=2$, find the product of $a$ and $b$ by using formulae.
Last edited by Phlembac Adib Hasan on Fri Oct 07, 2016 8:33 am, edited 1 time in total.
Reason: Latexed

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Phlembac Adib Hasan
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Re: Answer the question

Unread post by Phlembac Adib Hasan » Sun Oct 09, 2016 10:14 am

Welcome to BdMO Online Forum. Let me answer it since nobody is doing so.

Suppose $m=2x+1$. The product becomes $\begin{align*}(m+4)(m-4) &=m^2-16\\
&=(2x+1)^2-16\\
&=4x^2+4x-15
\end{align*}$
Now setting $x=2$, we get $16+8-15=9$
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Toky Tahmid Sami
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Re: Answer the question

Unread post by Toky Tahmid Sami » Sat Oct 15, 2016 10:05 pm

The product of $a$ and $b$ is,
\[\begin{split}
a\times b &= (2x-3)(2x+5)\\
&= \{2x+(-3)\}(2x+5)\\
&= (2x)(2x)+(5-3)2x-15\\
&= 16+8-15\\
&= 9
\end{split}\]
Last edited by Phlembac Adib Hasan on Mon Nov 07, 2016 12:38 pm, edited 1 time in total.
Reason: Latexed

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