[(x+2y)(x-2y)-(2x-y)^2+(3x-y)(2x-5y)]/(-1/2x)
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肯定对哦

[(x+2y)(x-2y)-(2x-y)^2+(3x-y)(2x-5y)]/(-1/2x)

=[(x^2-4y^2)-(4x^2-4xy+y^2)+(6x^2-17xy+5y^2)]/(-1/2x)

=(x^2-4y^2-4x^2-y^2+6x^2-17xy+5y^2)/(-1/2x)

=(3x^2-13xy)/(-1/2x)

=-6x+26y

79*3.14+10*1.34+11*1.34

=79*3.14+1.34*(10+11)

=79*3.14+1.34*21

=(100-21)*3.14+1.34*21

=314-21*3.14+1.34*21

=314-21*(3.14-1.34)

=314-21*1.8

=314-(20+1)*1.8

=314-36-1.8

=276.2