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dicky101618 2015-9-17 10:11 PM

數學問題log

2(3^(2x+1))+3^x-1=0

求x

hkpal 2015-9-17 11:26 PM

[quote]原帖由 [i]dicky101618[/i] 於 2015-9-17 10:11 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=425972410&ptid=25105520][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
2(3^(2x+1))+3^x-1=0

求x [/quote]

Just write it as 2[font=新細明體][size=12pt]•[font=Tahoma]3[font=新細明體][size=12pt]•[font=Tahoma](3^x)^2 + (3^x) - 1 = 0 and solve for (3^x) first.[/font][/size][/font][/font][/size][/font]

top11 2015-9-18 04:02 AM

[color=#0000ff]2(3^(2x+1)) + 3^x - 1 = 0[/color]
[color=#0000ff][/color]
[color=#0000ff][/color][color=#0000ff]2(3^(2x) × 3) + 3^x - 1 = 0[/color]

[color=#0000ff][/color][color=#0000ff][/color][color=#0000ff]6(3^x)² + 3^x - 1 = 0[/color]

[color=#0000ff][/color][color=#0000ff][/color][color=#0000ff][3(3^x) - 1][2(3^x) + 1] = 0[/color]

[color=#0000ff][/color][color=#0000ff][/color][color=#0000ff]3^x = 1/3  or  3^x = -1/2 (rejected)[/color]
[color=#ff0000][/color]
[color=#ff0000]x = -1[/color]

[color=#800080][/color][color=#0000ff][/color][color=#800080]〔不必用 log。〕[/color]

dicky101618 2015-9-19 06:03 PM

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