幫我解2題 中一/中二 MATHS 代數 方程 , 超急急急急急

我知你係邊個^^ 哇卡卡xD
當x係n

3ⁿ一2(3ⁿ-¹)=27ˉ
3(3ⁿ-¹)一2(3ⁿ-¹)=3³
3ⁿ-¹=3³
ⁿ-¹=³
x=4


第二條唔識=33=\\\

2008-07-20 21:29:58 補充：
第二條 當x=n

3ⁿ-¹=2430一3ⁿ+¹
3ⁿ-¹+3ⁿ+¹=2430
9(3ⁿ+¹)+3ⁿ+¹=2430
10(3ⁿ+¹)=2430
3ⁿ+¹=2430÷10
3ⁿ+¹=3既5次方
ⁿ+¹=5
n=4

9(3ⁿ+¹)知所以乘9係因為3ⁿ-¹比3ⁿ+¹少自乘兩次,SO3乘3=9....上一題亦是如此

但為何和kwan87_87的答案不一?!?!?!?

2008-07-20 21:35:55 補充：
好似我錯左=]

kwan87_87岩=]...........
第二條 當x=n

3ⁿ-¹=2430一3ⁿ+¹
3ⁿ-¹+3ⁿ+¹=2430
3ⁿ-¹+9(3ⁿ-¹)=2430
10(3ⁿ-¹)=2430
3ⁿ-¹=2430÷10
3ⁿ-¹=3既5次方
ⁿ-¹=5
n=6


9(3ⁿ-¹)知所以乘9係因為3ⁿ-¹比3ⁿ+¹少自乘兩次,SO 3乘3=9....then當3ⁿ+¹乘9就=9(3ⁿ-¹)上一題亦是如此

3^x - 2*3^(x-1) = 27
3^(x-1) [ 3 -2 ] =27
3^(x-1) = 27
3^(x-1) = 3^3
Therefore,
x-1 =3
x = 4


3^(x-1) = 2430 - 3^(x+1)
3^(x-1) + 3^(x+1) = 2430
3^(x-1) [ 1+3^2] = 2430
3^(x-1) [10]= 2430
3^(x-1) = 243
3^(x-1) = 3^5
x-1 =5
x = 6

解x =2,2=91. 3的x次方 減 2(3的x減1次方)=27