Need help on Math Hmwk !~~

VagueHart SG · 2007-01-27T13:21:00+00:00

i am sec 3 but i dont know wtm is lg. use the fomula: x= (-b +/-square root b*2 - 4(a)(c)) over 2(a)

VagueHart SG

Embarassed Sorry my math veh poor one..

Lets use * to denote the power of
EG 2*2 = 4 okk??

How to do the below question?

Solve the equation below giving your answers corrected to 3 significant figures
2lg(3x-2) = 5lg2.7

Izzit a quadratic equation in the end?

I do untill 9x*2-12x-139 = 0 den cannot do liao since 139 can only be formed by itself... >.<

Thanks in advance !~

the Bear SG

uhh.. 5lg2.7 = lg(2.7)*5 = some very huge number with 5 decimal places right?

VagueHart SG

err my calculator spoil so I use the microsoft one… I first type 2.7*5 which is 143.489 den lg it.. I got 2.1568

the Bear SG

why lg it?

remove the external multiplier on both sides so you get lg a = lg b

therefore a = b

then solve it using the formula for quadratic equations

VagueHart SG

Did that, i got the quad equation… its 9x*2-12x-139 which I think is impossible to do ask 139 is a number that can be divided only by itself and 1

Lorry` SG

i think he's sayin the end equation is 9x^2 - 12x - 139 = 0
and since 139 = 1* 139 only, then cannot solve, cant use cross method

hmm.. sec3 maths if i can rem ?
theres another method to solve? other than cross method,
maybe try the -b plusminus squareroot b^2 - 4ac over 2a method ? Laughing

VagueHart SG

oh ya... ok i go try now.. thanks Very Happy

the Bear SG

point: never lower it to 3 sig fig in the course of your calculations...

which means you have to get the whole mess of 5 decimal places until the last bit..

so you use this:

to solve..

hisoka SG

2lg(3x-2) = 5lg(2.7)

=>lg(3x-2) = lg((2.7)^(5/2))
therefore, 3x-2 = 2.y^(5/2)

Lorry` SG

actually i was wonderin, can straight away round off to a whole number meh?
9x^2 - 12x = 139.48907

the bear is right, should use the 139.48907 first, more accurate

seow SG

so free can teach math cannot go sbh crapbox. Mad Laughing

VagueHart SG

Embarassed

hehe thanks guys!~ done it Laughing thanks alot!

Lorry` SG

hisoka's method also can be used, it's faster, but in terms of accuracy im not sure which's better

Laughing

the Bear SG

it's almost the same for the basic level...

you kind of lose a pile of stuff like complex numbers because lg(-ve number) isn't on the natural number line or something like that Laughing

VagueHart SG

why hisoka’s method got y de?? typo?? sorry my math really veh terrible de

the Bear SG

type wrongly.. notice y is near 7

should be 2.7

hisoka SG

2.7 sorry typo, the metohd mroe important lah

SGpork SG

wa this is getting famous

everyone post their hmw

Laughing

Lorry` SG

hmm.... ok
are you a maths teacher? Smile

the Bear SG

nope.. last time i did math homework was in JC.. which is more than half a lifetime ago Laughing