中一數學 (sequence)

a sequence has general term 7-n.
-find the 100th term of the sequence.
the 100th term of the sequence = 7-100 = -93

-which term of the sequence is equal to 0? 7

-how many ? 7-n >or= 0
7 >or= n
n<or= 7
the answer is between 1-7 , so there are 7 terms of sequence are positive.

one number is 18 greater than another number. if the smaller number is subtracted from three times the greater number, the result is 78. find the sum of the two number

let the small number be x and the greater one be x+18

3(x+18) - x =78
3x+54 -x =78
2x = 78-54
x = 12
the sum of the two number = 12+12+18= 42

when a number is increased by 32% and then multiplied by 80%, the result is 1.9008. find the number..

let the number be a

a*(1+32%)*80% =1.9008
a = 1.9008/1.32/0.8=1.8

each apple costs $m and each orange costs $n. Ben buys 3 apples and 4 oranges and pays $20. How much change should be given?

$(20-3m-4n)

2008-08-11 00:46:17 補充：
how many terms of sequence are positive?

I am sorry to say that I have to correct this part.

I've just remembered that 0 is neither positive nor negative.

So, n-7 > 0
n < 7
the anwser is between 1-6 , so there are 6 terms of sequence are positive.

5,3,1,-1

100 th of the sequence

7-100
= -93

which term of the sequence is equal to 0

7-n = 0
-n =7

7-7=0. n =7 so the 7 terms

how many terms of sequence are positive

i) 7-1=6
........
v) 7-5=2
vi)7-6=1

so 6 terms of sequence are positive

the other question i don't know am i correct

18 is bigger then a no. we call ?

18x3-? =78
54-? =78
-?= 24
?=-24

sum 18+(-24) = 18-24 = -6