李二猫
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共回答了21个问题采纳率:85.7% 举报
a² - 5a + 1 = 0
根据方程式,
x = [-b ± √(b² - 4ac)] / 2a
a = [5 ± √(25 - 4)] / 2
a = (5 ± √21) / 2
当 a = (5 + √21) / 2
(a - 1)/a
= {[(5 + √21) / 2] - 1} / [(5 + √21) / 2]
= (3 + √21) / (5 + √21)
= 0.79
当 a = (5 - √21) / 2
(a - 1)/a
= {[(5 - √21) / 2] - 1} / [(5 - √21) / 2]
= (3 - √21) / (5 - √21)
= -3.79
1年前
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5
无兄弟不篮球
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我已经知道答案了,而且我问的是是a-1/a 不是(a - 1)/a
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李二猫
当 a = (5 + √21) / 2 a - 1/a = [(5 + √21) / 2] - 1 / [(5 + √21) / 2] = [(5 + √21) / 2] - [2 / (5 + √21)] = (25 + 10√21 + 21 - 2) / 2(5 + √21) = (44 + 10√21) / 2(5 + √21) = 4.687 ( 3 d.p. ) 当 a = (5 - √21) / 2 a - 1/a = [(5 - √21) / 2] - 1 / [(5 - √21) / 2] = [(5 - √21) / 2] - [2 / (5 - √21)] = (25 - 10√21 + 21 - 2) / 2(5 - √21) = (44 - 10√21) / 2(5 - √21) = -0.381 ( 3 d.p. )