a. ( a - b - 2c ) - ( a - b - c )
= a - b - 2c - a + b + c
= ( a - a ) + ( b - b ) + ( 2c + c )
= 0 + 0 + 3c
= 3c.
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b. ( 9 - 2x )( x2 - 25 ) = 0.
⇒ \(\left[{}\begin{matrix}9-2x=0\\x^2-25=0\end{matrix}\right.\)
⇒ \(\left[{}\begin{matrix}2x=9\\x^2=25\end{matrix}\right.\)
⇒ \(\left[{}\begin{matrix}x=4,5\\x=5\end{matrix}\right.\)
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