Bài làm
B = ( x - 9 ) . ( 2x + 3 ) - 2 . ( x + 7 ) . ( x - 5 )
B = 2x2 + 3x - 18x - 27 - ( 2x + 14 )( x - 5 )
B = 2x2 + 3x - 18x - 27 - ( 2x2 - 10x + 14x - 70 )
B = 2x2 + 3x - 18x - 27 - 2x2 - 10x + 14x - 70
B = ( 2x2 - 2x2 ) + ( 3x - 18x - 10x + 14x ) + ( -27 + 70 )
B = -11x + 43
Vậy B = -11x + 43
B = ( x - 9 )( 2x + 3 ) - 2( x + 7 )( x - 5 )
B = 2x2 - 15x - 27 - 2( x2 + 2x - 35 )
B = 2x2 - 15x - 27 - 2x2 - 4x + 70
B = -19x + 43
Bài làm:
\(B=\left(x-9\right)\left(2x+3\right)-2\left(x+7\right)\left(x-5\right)\)
\(B=2x^2-15x-27-2\left(x^2+2x-35\right)\)
\(B=2x^2-15x-27-2x^2-4x+70\)
\(B=-19x+43\)
Ta có: \(B=\left(x-9\right)\left(2x+3\right)-2\left(x+7\right)\left(x-5\right)\)
<=>\(B=\left(2x^2-15x-27\right)-2\left(x^2+2x-35\right)\)
<=>\(B=-19x+43\)
Còn lại c tính gía trị của B thì thay x vào để tính...
Chúc c học tốt!
Trả lời:
Ta có:\(B=0\)
\(\Rightarrow\left(x-9\right).\left(2x+3\right)-2.\left(x+7\right).\left(x-5\right)=0\)
\(\Leftrightarrow2x^2+3x-18x-27-2.\left(x^2-5x+7x-35\right)=0\)
\(\Leftrightarrow2x^2-15x-27-2.\left(x^2+2x-35\right)=0\)
\(\Leftrightarrow2x^2-15x-27-2x^2-4x+70=0\)
\(\Leftrightarrow-19x+43=0\)
\(\Leftrightarrow-19x=-43\)
\(\Leftrightarrow x=\frac{43}{19}\)
Vậy \(x=\frac{43}{19}\)
\(\Leftrightarrow x=\frac{-43}{19}\)
