# Thread: 2U Maths sequence & series help!

1. ## 2U Maths sequence & series help!

17-20 in Maths in Focus Mathematics hsc book
17. the first negative term of the series tn=1000-2n
18. The first positive term of the series tn=2n-300
19.a) the first negative term of the series un=80-6n
b) the first negative term
20. The first negative term of the series tn=50-7n

2. ## Re: 2U Maths sequence & series help!

solve $T_n$ with respect to the inequality

for example, question 17:

$T_n = 1000-2n$

$1000-2n < 0$

$1000 < 2n$

$n > 500$

so the first negative term will be the 501th term

this is similar for all questions you have listed

so using the same concept as above, Q18: 151th term, Q19: 14th Term and Q20: 8th term

this can also be used by subbing the number into the equation to see wheather it gives you a positive/negative term respectively

also don't worry about recieving a fractional answer when you solve the equation. Just round it up if the answer asks for greater. The reason we round it up is because we want the first integer that is greater than the inequality

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