Question

解方程：x²＋2x-15＝0.

Answer 1

解法一：a="1," b="2," c="-15,"
>0.           …………………………………………2分
                      …………………………………………3分
∴x1 =" 3," x2 =" -5.                      " …………………………………………5分
解法二：( x -3 )( x+5 )＝0，                   …………………………………………3分
∴x1 =" 3," x2 =" -5.                      " …………………………………………5分
解法三：x2＋2x＝15，
x2＋2x+1＝15+1.                    …………………………………………2分
(x＋1)2＝42.                        …………………………………………3分
x＋1＝±4.
∴x1 =" 3," x2 =" -5.                      " …………………………………………5

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