Posted by Lexie on Thursday, April 22, 2010 at 10:25pm.
Solve the quadratic equation.
3x^215x+18=0
I know that the formula to quadratic equation is x=b+and(square root)b^24ac divided by 2a.
I know that a=3, b=15 and c=18, then subsitute in and get two answers, my problem is that I keep getting an answer that does not work after I subsitute in the original equation. What do I do?

Algebra  Reiny, Thursday, April 22, 2010 at 10:52pm
did you get
x = (15 ± √(225  4(3)(18) )/6 ?
= (15 ± √9)/6
= (15 ±3)/6
= 3 or 2
we could have divided the original equation by 3 to get
x^2  5x + 6 = 0
now it factors
(x3)(x2) = 0
so x = 3 or x = 2

Algebra  Lexie, Thursday, April 22, 2010 at 11:01pm
I got 3 and 2. But to check it, wouldn't you substitue it the 3 and then the 2 in the original equation and it should be 0=0?

Algebra  Reiny, Friday, April 23, 2010 at 6:46am
and so it does
How did you get 2?
if you had a + and a  then you couldn't possible end up with +18 at the end.
if x= 3
3(9)  45 + 18 = ??
if x=2
3(42)  30 + 18 = ???
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