Question

已知x+y=5,x-y=3,求:x^2-y^2 x^2+y^2 xy的结果

Answer 1

x^2-y^2 =(x+y)(x-y) =5*3 =15 x^2+y^2 =1/2(x+2xy+y+x-2xy+y)] =1/2[(x+y)+(x-y)] =1/2(25+9) =17 xy =1/4[(x+2xy+y)-(x-2xy+y)] =1/4[(x+y)-(x-y)] =1/4(25-9) =4