simplifying and factorising question driving me crazy!?

2008-09-21 9:17 am

ok so i (thought) i factorised & simplifyed these as much as possible
a) s^2-s-20 / s^2 + 4 s
b) s^2-s-20 / s^2 + 5 s

and got a&b to be:
a) (s-5) / s
and
b) (s-5)(s+4)/s*(s+5)

this is for an online math quiz and it said this:
INCORRECT!
Hint: both the numerator and denominator are quadratics in s that can be factored over the integers (i.e. the factors should only involve integers).

where am i going wrong?? :S:S

更新1:

got part b to enter correctly... but still having trouble with a :(:(:(

更新2:

got them both to be correct... thanks guys

回答 (6)

[匿名]

2008-09-21 9:28 am

✔ 最佳答案

a) (s^2-s-20 )/ (s^2 + 4 s)
=(s+4)(s-5)/(s(s+4))
=(s-5)/s

b) (s^2-s-20) / (s^2 + 5 s)

=(s-5)(s+4)/ (s(s+5))

but in the case
(s^2+s-20) / (s^2 + 5 s)
=(s+5)(s-4)/ (s(s+5))
=(s-4)/s

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An ESL Learner

2008-09-21 5:20 pm

a)
(s^2 - s - 20)/(s^2 + 4s)
= (s^2 + 4s - 5s - 20)/s(s + 4)
= (s + 4)(s - 5)/s(s + 4) (cancel out s + 4)
= (s - 5)/s
= s/s - 5/s
= 1 - 5/s

b)
(s^2 - s - 20)/(s^2 + 5s)
= (s^2 + 4s - 5s - 20)/s(s + 5)
= (s + 4)(s - 5)/s(s + 5)

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Como

2008-09-21 4:40 pm

Question a)
(s - 5)(s + 4)
-------------------
s (s + 4)

(s - 5)
-----------
s

Question b)
(s - 5) (s + 4)
-----------------
s (s + 5)

Suggest that question has been presented incorrectly as nothing cancels.

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[匿名]

2008-09-21 4:31 pm

a) (s^2 - s - 20) : (s^2 + 4s) =
....Factorize:
....(s - 5) (s + 4) : s (s + 4) =
....Eliminate:
....(s - 5) : s
....Simplify:
....1 - 5/s ---------- final answer

b) - (1+1/s) (7 + 15/s) (1/5+s)

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[匿名]

2008-09-21 4:31 pm

My approach would be long division on equations.

Try reading this website and see if it'll give you any insight: http://www.sosmath.com/algebra/factor/fac01/fac01.html

a) 1 + (-5s - 20)/(s^2 + 4s)
b) 1 + (-6s - 20)/(s^2 + 5s)

參考: http://www.sosmath.com/algebra/factor/fac01/fac01.html

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[匿名]

2008-09-21 4:29 pm

That is impossible. You cant get only integers as answers. Or maybe my knowledge in maths in adequate.

PS: I get highest percentage in maths in my college.

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