(a+b)^2=49,(a-b)^2=21 则ab=
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(a+b)^2=49,(a-b)^2=21 则ab=
(a+b)^2=49,(a-b)^2=21 则ab=
(a+b)^2=49,(a-b)^2=21 则ab=
展开两式
a^2+b^2+2ab=49
a^2+b^2-2ab=21
两式相减
4ab=28
ab=7