已知:a+b=7,ab=-3,则a²+b²的值是?A.55 B.43 C.20 D.13
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已知:a+b=7,ab=-3,则a²+b²的值是?A.55 B.43 C.20 D.13
已知:a+b=7,ab=-3,则a²+b²的值是?
A.55 B.43 C.20 D.13
已知:a+b=7,ab=-3,则a²+b²的值是?A.55 B.43 C.20 D.13
a²+b²
=a²+2ab+b²-2ab
=(a+b)²-2ab
将a+b=7,ab=-3代入上式
得:原式=7²-2×(-3)
=55
故选a
a²+b²
=a²+2ab+b²-2ab
=(a+b)²-2ab
=7²-2*(-3)
=55
a²+b²
=a²+2ab+b²-2ab
=(a+b)²-2ab
=7²-2*(-3)
=55
所以,选A
因为(a+b)²=a²+2ab+b²=7²=49,由ab=-3
所以a²+b²=49-2ab=49+6=55
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