give a, b and c are rational numbers, show the expressions are rational.

i.) a^2 + b^2 + c ^2

ii.) (5a^3 +6b^4)/(a^2+b^2)

iii.) a + a^2 + a^3 + ... +a^10

I had alot of trouble proving this, and it was not enough work.

please explain these.

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- March 5th 2008, 08:39 PMrcmangoshow the expressions are rational numbers
give a, b and c are rational numbers, show the expressions are rational.

i.) a^2 + b^2 + c ^2

ii.) (5a^3 +6b^4)/(a^2+b^2)

iii.) a + a^2 + a^3 + ... +a^10

I had alot of trouble proving this, and it was not enough work.

please explain these. - March 6th 2008, 09:25 AMCaptainBlack
- March 6th 2008, 01:34 PMiknowone
Speaking generally there are a lot of different operations one can do to rational numbers without leaving the rational world.

The product of any finite number of rational numbers is rational:

let be rational numbers.

say are integers.

then are integers

therefore is rational.

Same kind of proofs can be used to show the sum of 2 rationals is rational, which extends inductively to the finite sum of rationals. The division of rationals remaining rational is a direct consequence of the multiplication of rationals remaining rational. So it is not hard to see that any finite composition of such operations on rationals results in a rational. Proving these general statements might be easier than specific examples which can get very messy. - March 9th 2008, 02:44 PMrcmango
iii.) a + a^2 + a^3 + ... +a^10

i could let a = f/e

then = (f/e)^2

then no matter what to the 10th power it is rational.

is that correct?