已知A=5x的平方y-3xy的平方+4xyB=7xy的平方-2xy+x的平方y(1)A-2B=?(2)若A+B+2C=0,求C的值
问题描述：
已知A=5x的平方y-3xy的平方+4xyB=7xy的平方-2xy+x的平方y
(1)A-2B=?(2)若A+B+2C=0,求C的值
答
A = 5x²y - 3xy² + 4xy
B = 7xy² - 2xy + x²y
(1)
A - 2B
= (5x²y - 3xy² + 4xy) - 2(7xy² - 2xy + x²y)
= 5x²y - 3xy² + 4xy - 14xy² + 4xy - 2x²y
= 3x²y - 17xy² + 8xy
(2)
A + B + 2C = 0
C = ( - A - B)/2
= (-5x²y + 3xy² - 4xy - 7xy² + 2xy - x²y)/2
= (-6x²y - 4xy² - 2xy)/2
= -3x²y - 2xy² - xy
答
A = 5x²y - 3xy² + 4xyB = 7xy² - 2xy + x²y(1)A - 2B= (5x²y - 3xy² + 4xy) - 2(7xy² - 2xy + x²y)= 5x²y - 3xy² + 4xy - 14xy² + 4xy - 2x²y= 3x...