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Ta có :
\(A+B+C=4x^2-5xy+5y^2+3x^2+2xy+y^2+x^2+3xy+2y^2\)
\(=\left(4x^2+3x^2+x^2\right)+\left(-5xy+2xy+3xy\right)+\left(5y^2+y^2+2y^2\right)\)
\(=8x^2+8y^2\)
\(B-C-A=\left(3x^2+2xy+y^2\right)-\left(x^2+3xy+2y^2\right)-\left(4x^2-5xy+5y^2\right)\)
\(=3x^2+2xy+y^2-x^2-3xy-2y^2-4x^2+5xy-5y^2\)
\(=\left(3x^2-x^2-4x^2\right)+\left(2xy-3xy+5xy\right)+\left(y^2-2y^2-5y^2\right)\)
\(=-2x^2+4xy-6y^2\)
\(C-A-B=\left(x^2+3xy+2y^2\right)-\left(4x^2-5xy+5y^2\right)-\left(3x^2+2xy+y^2\right)\)
\(=x^2+3xy+2y^2-4x^2+5xy-5y^2-3x^2-2xy-y^2\)
\(=\left(x^2-4x^2-3x^2\right)+\left(3xy+5xy-2xy\right)+\left(2y^2-5y^2-y^2\right)\)
\(=-6x^2+6xy-4y^2\)
\(A+B-C=4x^2-5xy+5y^2+3x^2+2xy+y^2-\left(x^2+3xy+2y^2\right)\)
\(=4x^2-5xy+5y^2+3x^2+2xy+y^2-x^2-3xy-2y^2\)
\(=\left(4x^2+3x^2-x^2\right)+\left(-5xy+2xy-3xy\right)+\left(5y^2+y^2-2y^2\right)\)
\(=6x^2-6xy+4y^2\)
Bài 26:
\(A+B+C=4x^2-5xy+3y^2+3x^2+2xy+y^2-x^2+3xy+2y^2\)
\(=\left(4x^2+3x^2-x^2\right)+\left(-5xy+2xy+3xy\right)+\left(3y^2+y^2+2y^2\right)\)
\(=6x^2+6y^2\)
\(B-C-A=\left(3x^2+2xy+y^2\right)-\left(-x^2+3xy+2y^2\right)-\left(4x^2-5xy+3y^2\right)\)
\(=3x^2+2xy+y^2+x^2-3xy-2y^2-4x^2+5xy-3y^2\)
\(=\left(3x^2-4x^2+x^2\right)+\left(2xy-3xy+5xy\right)+\left(y^2-2y^2-3y^2\right)\)
\(=-4xy-2y^2\)
\(C-A-B=\left(-x^2+3xy+2y^2\right)-\left(4x^2-5xy+3y^2\right)-\left(3x^2+2xy+y^2\right)\)
\(=-x^2+3xy+2y^2-4x^2+5xy-3y^2-3x^2-2xy-y^2\)
\(=\left(-x^2-4x^2-3x^2\right)+\left(3xy+5xy-2xy\right)+\left(2y^2-3y^2-y^2\right)\)
\(=-8x^2+6xy-2y^2\)
cái câu B-C-A ý thì kết quả phải là 4xy-4y^2 chứ
vì: 2xy-3xy+5xy =4 xy
y^2 - 2y^2-3y^2 = -4y^2
=> = 4xy-4y^2
a, (3x2-2xy+y2) + (x2-xy+2y2) - (4x2-y2)
= 3x2-2xy+y2+x2-xy+2y2-4x2+y2
= 4y2-3xy
b, = x2-y2+2xy-x2-xy-2y2+4xy-1
= -3y2+5xy
c, M=5xy+x2-7y2+(2xy-4y)2 = 5xy+x2-7y2+4x2y2-16xy2+16y2 = 5xy+x2+9y2+4x2y2-16xy2
A + B + C = ( 4x2 - 5xy + 3y2 ) + ( 3x2 + 2xy + y2 ) + ( -x2 + 3xy + 2y2 )
= 4x2 - 5xy + 3y2 + 3x2 + 2xy + y2 - x2 + 3xy + 2y2
= ( 4x2 - 3x2 - x2 ) + ( 5xy + 2xy + 3xy ) + ( 3y2 + y2 + 2y2 )
= 10xy + 6y2
B - C - A = ( 3x2 + 2xy + y2 ) - ( -x2 + 3xy + 2y2 ) - ( 4x2 - 5xy + 3y2 )
= 3x2 + 2xy + y2 + x2 - 3xy - 2y2 - 4x2 + 5xy - 3y2
= ( 3x2 + x2 - 4x2 ) - ( 2xy - 3xy + 5xy ) - ( y2 - 2y2 - 3y2 )
= 0 - 0 - ( -4y2 )
= 0 + 4y2
= 4y2
C - A - B = ( -x2 + 3xy + 2y2 ) - ( 4x2 - 5xy + 3y2 ) - ( 3x2 + 2xy + y2 )
= -x2 + 3xy + 2y2 - 4x2 + 5xy - 3y2 - 3x2 - 2xy - y2
= ( -x2 - 4x2 - 3x2 ) - ( 3xy + 5xy - 2xy ) - ( 2y2 - 3y2 - y2 )
= -8x2 - 6xy - ( -2y2 )
= -8x2 - 6xy + 2y2
* Lần đầu làm xD Sai sót gì mong bạn thông cảm *
Trl :
bạn kia làm đúng rồi nhé
hk tốt nhé bạn @