已知:(a+b)² =7,(a-b)² =4,求a² +b² 和ab的值.
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已知:(a+b)² =7,(a-b)² =4,求a² +b² 和ab的值.
已知:(a+b)² =7,(a-b)² =4,求a² +b² 和ab的值.
已知:(a+b)² =7,(a-b)² =4,求a² +b² 和ab的值.
(a+b)² =a²+2ab+b²=7 式(1)
(a-b)²=a²-2ab+b²=4 式(2)
(1)+(2)=2(a² +b² )=4+7=11 所以a² +b² =11/2=5.5
(1)—(2)=4ab=3 所以ab=3/2=1.5