(1)已知x+y=3,x²+y²=7,求xy的值 (2)已知a+2b=5,求(a-2b)²的值

问题描述:

(1)已知x+y=3,x²+y²=7,求xy的值 (2)已知a+2b=5,求(a-2b)²的值
第二题 已知a+2b=5,ab=2

【1】(x+y)² = x²+y²+2xy所以 xy = [(x+y)²-x²-y²]/2 = (3²-7)/2 = 1【2】条件不全,应该有给出ab的值吧,然后用公式:(a-2b)² = (a+2b)²-8ab第二题 已知a+2b=5,ab=2(a-2b)² = (a+2b)²-8ab = 5²-8×2 = 9