SAT数学题求解答
Ifeachtermofthesequence(a1,a2,......,an)iseither7or77,andthesumofthesequenceis350.Whi...
If each term of the sequence (a1,a2,......,an) is either 7 or 77, and the sum of the sequence is 350. Which of following could be the value of n?
A 45 B 42 C 40 D 39 E 36
不太理解这道题什么意思,求讲解 展开
A 45 B 42 C 40 D 39 E 36
不太理解这道题什么意思,求讲解 展开
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直译:如果数列{an}中每一项要么是7要么是77,且数列的(n项)和为350,那么(数列的项数)n的值可能是多少?
解答:设有a项为7,b项为77,已知7a+77b=350,求a+b的可能值.
用线性不定方程的知识可以算出a+b=50-10d,其中d>=0,且d<=4,那么当然选c了.
(更土的办法如下:已知条件为a+11b=50,b=0则a=50那么a+b=50;b=1则a=39那么a+b=40...b=4则a=6那么a+b=10,题中只有40可选。)
解答:设有a项为7,b项为77,已知7a+77b=350,求a+b的可能值.
用线性不定方程的知识可以算出a+b=50-10d,其中d>=0,且d<=4,那么当然选c了.
(更土的办法如下:已知条件为a+11b=50,b=0则a=50那么a+b=50;b=1则a=39那么a+b=40...b=4则a=6那么a+b=10,题中只有40可选。)
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如果数列(a1,a2,…,an)中每个数是7或77,且它们的和为350。则n的值可能是下列数字中的哪个?
C,7×39+77=350
因为它们的和是10的倍数且它们都有一个7,所以就选一个是10的倍数的答案
如满意请采纳^v^
C,7×39+77=350
因为它们的和是10的倍数且它们都有一个7,所以就选一个是10的倍数的答案
如满意请采纳^v^
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嘿嘿。。。
Answer: C.
Hope it helps.
Let x and y be the number of term 7 and the number of term 77 respectively. WE have :
7x+ 77y= 350 ---> x+11y = 50 ---> x+y= 50- 10y
we have : x+y= n = 50- 10y ---> n must have the unit digit of 0 because 50 - 10y must be a number which has unit digit of 0
Among the answer choices provided, only C matches.
I go for C.
Answer: C.
Hope it helps.
Let x and y be the number of term 7 and the number of term 77 respectively. WE have :
7x+ 77y= 350 ---> x+11y = 50 ---> x+y= 50- 10y
we have : x+y= n = 50- 10y ---> n must have the unit digit of 0 because 50 - 10y must be a number which has unit digit of 0
Among the answer choices provided, only C matches.
I go for C.
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