Toán Lớp 8: 1 Tìm x
a, 3x . ( 5x – 6 ) – 7x . ( 2x – 3 ) = 4
b, ( x ² – 3x + 9 ) – x. (x+ 1) = 25
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Toán Lớp 8: 1 Tìm x
a, 3x . ( 5x – 6 ) – 7x . ( 2x – 3 ) = 4
b, ( x ² – 3x + 9 ) – x. (x+ 1) = 25
Comments ( 2 )
a)
3x . ( 5x – 6 ) – 7x . ( 2x – 3 ) = 4
<=>15x^2-18x-14x^2+21x=4
<=>(15x^2-14x^2)+(21x-18x)-4=0
<=>x^2+3x-4=0
<=>x^2+4x-x-4=0
<=>x.(x+4)-(x+4)=0
<=>(x-1)(x+4)=0
<=>[(x-1=0),(x+4=0):}
<=>[(x=1),(x=-4):}
Vậy phương trình có tập nghiệm S={1;-4}
b)
(x^2-3x+9)-x.(x+1)=25
<=>x^2-3x+9-x^2-x=25
<=>(x^2-x^2)-(3x+x)+9=25
<=>-4x+9-25=0
<=>-4x-16=0
<=>-4x=16
<=>x=-4
Vậy phương trình có nghiệm S={-4}