# Solving Equations

• Jul 31st 2010, 04:37 AM
KelvinScale
Solving Equations
Hi everyone,

I was working on some solving equations in preparation for university, and came across a few that I just could not understand how to do them. I have the answers, but do not know how to get to them, how painful....

1. Find a cubic function of x whose graph has x-intercepts equal to -1 and 2, and in which f(0)=6 and f(3)=12.

2. The roots of the equation x^2+px+1=0 are a and b, and the roots of the equation x^2-9x+q=0 are a+2b and 2a+b. (a and b are greek symbols).

3. If the roots of the equation x^3-15x^2+cx-105, are a, a+b, and a-b, determine the values of a,b, and c.

• Jul 31st 2010, 04:48 AM
Prove It
3. \$\displaystyle x^3 - 15x^2 + cx - 105\$ has roots \$\displaystyle x = a, x = a+ b, x = a - b\$.

That means \$\displaystyle f(a) = 0, f(a + b) = 0, f(a - b) = 0\$.

So \$\displaystyle a^3 - 15a^2 + ac - 105 = 0\$
\$\displaystyle (a+b)^3 - 15(a+b)^2 + (a+b)c - 105 = 0\$
\$\displaystyle (a-b)^3 - 15(a-b)^2 + (a-b)c - 105 = 0\$.

Solve these three equations simultaneously for \$\displaystyle a,b,c\$.
• Jul 31st 2010, 04:51 AM
mr fantastic
Quote:

Originally Posted by KelvinScale
Hi everyone,

I was working on some solving equations in preparation for university, and came across a few that I just could not understand how to do them. I have the answers, but do not know how to get to them, how painful....

1. Find a cubic function of x whose graph has x-intercepts equal to -1 and 2, and in which f(0)=6 and f(3)=12.

[snip]

Use the model \$\displaystyle f(x) = a(x + 1)(x - 2)(x - b)\$ (why?) and use the fact that f(0) = 6 and f(3) = 12 to get two simuatenous equations for a and b.

Quote:

Originally Posted by KelvinScale
[snip]
2. The roots of the equation x^2+px+1=0 are a and b, and the roots of the equation x^2-9x+q=0 are a+2b and 2a+b. (a and b are greek symbols).

[snip]

\$\displaystyle (x - a)(x - b) = x^2 + px + 1\$. Therefore:

\$\displaystyle ab = 1\$ .... (1)

\$\displaystyle a + b = -p\$ .... (2)

In a similar way, \$\displaystyle (x - a - 2b) (x - 2a - b) = x^2 - 9x + q\$. Therefore:

...... (3)

....... (4)

Solve equations (1), (2), (3) and (4) simultaneously to get p and q.

If you need more help, please show all your work and say where you get stuck.
• Jul 31st 2010, 03:30 PM
KelvinScale