天山07_20

[ai12345]

20.if 2^x+2^x+2^x+2^x=2^n,what is x in terms of n?
A.n/4
B.4n
C.2n
D.n-2
E.n+2
答案Dn-2
搞不清楚.
應該是B吧
請前輩幫忙解答.thanks

[FrankChang]

2^x+2^x+2^x+2^x=2^n,
=>4*(2^x)=2^n,
=>2^(2+x)=2^n,
=>2+x=n,
=>x=n-2.

[StanleyHuang]

有四個2^n相加，所以原式 = 4*(2^x) = (2^2)*(2^x) = 2^(x+2) = 2^n
所以x = n-2，選(D)