己知a-b=5,ab=-1,求a和b的值则3a-3(ab+b)的值是?

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己知a-b=5,ab=-1,求a和b的值则3a-3(ab+b)的值是?
己知a-b=5,ab=-1,求a和b的值
则3a-3(ab+b)的值是?

己知a-b=5,ab=-1,求a和b的值则3a-3(ab+b)的值是?
a - b = 5
ab = - 1
b = a - 5
a(a - 5) = - 1
a² - 5a + 1 = 0
a² - 5a + 25/4 = 21/4
(a - 5/2)² = 21/4
a - 5/2 = ± √21 / 2
a = 5/2 ± √21 / 2
b = a - 5 = - 5/2 ± √21 / 2
3a-3(ab+b)
= 3(a - b) - 3ab
= 15 + 3
= 18

因为a-b=5,ab=-1
所以a=5+b
又因为ab=-1
5b+b平方+1=0
所以b=【-5±根号21】/2
当b=【-5+根号21】/2
a=-2/【-5+根号21】
当b=【-5-根号21】/2
a=2/【5+根号21】
3a-3(ab+b)=3(a-b)-3ab=15+3=18