pp1-ps-Q155,Q178,Q 180

[mupeane]

155. 16103-!-item-!-187;#058&010667
If 2^x - 2^(x - 2) = 3(2^13), what is the value of x ?

(A) 9
(B) 11
(C) 13
(D) 15
(E) 17

ANS:D

178. 18885-!-item-!-187;#058&011972
Adam and Beth each drove from Smallville to Crown City by different routes. Adam drove an an average speed of 40 miles per hour and completed the trip in 30 minutes. Beth's route was 5 miles longer, and it took her 20 minutes more than Adam to complete the trip. How many miles per hour was Beth's average speed on this trip?

(A) 24
(B) 30
(C) 48
(D) 54
(E) 75

ANS:B

180. 19628-!-item-!-187;#058&012404
List K consists of 12 consecutive integers. If -4 is the least integer in list K, what is the range of the positive integers in list K ?

(A) 5
(B) 6
(C) 7
(D) 11
(E) 12
ANS:B

有些答案我覺得很奇怪，請大家幫幫忙喔

[amazingslim]

155.
2^x - 2^(x - 2) = 3(2^13)
=>2^(x-2)(2^2-1)=3(2^13)
=>x-2=13
=>x=15

178.
A的距離=40*1/2=20
B的距離=20+5=25
B的速率=25/(50/60)=30

180.
數列K=-4,-3....,0,1,....4,5,6,7
正數範圍=7-1=6

[mupeane]

155.
2^x - 2^(x - 2) = 3(2^13)
=>2^(x-2)(2^2-1)=3(2^13)
=>x-2=13
=>x=15

這題式子可以再解釋詳細一點嗎?
請問等式右邊的3怎麼就不見了?

謝謝喔
我數學不大好

[amazingslim]

式子左邊:2的平方-1=3
可以跟右邊的3消除

[mupeane]

謝謝你喔
我終於知道了

[pimi]

對180題還是有小小不了解
正數的範圍1234567
不是有七個數嗎
用7-1不是會把其中一個數剪掉嗎？？
答案為何不是7而是6呢？？
請大大解答

[Philosophia]

對180題還是有小小不了解
正數的範圍1234567
不是有七個數嗎
用7-1不是會把其中一個數剪掉嗎？？
答案為何不是7而是6呢？？
請大大解答

因為這題是在問 range，所以是 6
但問題若是有多少正數，才會是 7