9x^3-18x=x-2 equation
The teacher will be very surprised to see your correct solution 😉
The solution
Vieta's Theorem
rewrite the equation
9x3−18x=x−2of
ax3+bx2+cx+d=0as reduced cubic equation
x3+abx2+acx+ad=0x3−919x+92=0px2+qx+v+x3=0where
p=abp=0q=acq=−919v=adv=92Vieta Formulas
x1+x2+x3=−px1x2+x1x3+x2x3=qx1x2x3=vx1+x2+x3=0x1x2+x1x3+x2x3=−919x1x2x3=92
The graph
Sum and product of roots
[src]
/ 1 \ / / 1 \ \ / 1 \ / / 1 \ \
19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\ 19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\
| __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 || | __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 || __________________
| / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------|| | / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------|| / ______
| / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /| | / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /| / 2*I*\/ 4962
|3 / 3 + ------------ *|- - - -------|| | |3 / 3 + ------------ *|- - - -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------| |3 / 3 + ------------ *|- - + -------|| | |3 / 3 + ------------ *|- - + -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------| 3 / 3 + ------------
\\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 / \\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 / 19 \/ 9
- ---------------------------------------------- + I*|- ---------------------------------------------- + ---------------------------- + ----------------------------| - ---------------------------- + ---------------------------- + - ---------------------------------------------- + I*|- ---------------------------------------------- - ---------------------------- + ----------------------------| + ---------------------------- + ---------------------------- + - ------------------------- - -----------------------
9 \ 9 6 18 / 6 18 9 \ 9 6 18 / 6 18 __________________ 3
/ ______
/ 2*I*\/ 4962
9*3 / 3 + ------------
\/ 9
−619sin(3atan(2724962))−919re((−21−23i)33+924962i1)+1857cos(3atan(2724962))+i−919im((−21−23i)33+924962i1)+1857sin(3atan(2724962))+619cos(3atan(2724962))+619sin(3atan(2724962))+1857cos(3atan(2724962))−919re((−21+23i)33+924962i1)+i−619cos(3atan(2724962))+1857sin(3atan(2724962))−919im((−21+23i)33+924962i1)+−333+924962i−933+924962i19
/ 1 \ / 1 \ / / 1 \ \ / / 1 \ \
19*re|---------------------------------------| 19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\
| __________________ | | __________________ | __________________ | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 ||
| / ______ / ___\| | / ______ / ___\| / ______ | | / ______ / ___\| |atan|----------|| |atan|----------||| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------||
| / 2*I*\/ 4962 | 1 I*\/ 3 || | / 2*I*\/ 4962 | 1 I*\/ 3 || / 2*I*\/ 4962 | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /|
|3 / 3 + ------------ *|- - + -------|| |3 / 3 + ------------ *|- - - -------|| 3 / 3 + ------------ | |3 / 3 + ------------ *|- - + -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| | |3 / 3 + ------------ *|- - - -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 57 *cos|----------------|
19 \\/ 9 \ 2 2 // \\/ 9 \ 2 2 // \/ 9 | \\/ 9 \ 2 2 // \ 3 / \ 3 /| | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 /
- ------------------------- - ---------------------------------------------- - ---------------------------------------------- - ----------------------- + I*|- ---------------------------------------------- - ---------------------------- + ----------------------------| + I*|- ---------------------------------------------- + ---------------------------- + ----------------------------| + ----------------------------
__________________ 9 9 3 \ 9 6 18 / \ 9 6 18 / 9
/ ______
/ 2*I*\/ 4962
9*3 / 3 + ------------
\/ 9
−919re((−21−23i)33+924962i1)−919re((−21+23i)33+924962i1)+957cos(3atan(2724962))−333+924962i+i−619cos(3atan(2724962))+1857sin(3atan(2724962))−919im((−21+23i)33+924962i1)+i−919im((−21−23i)33+924962i1)+1857sin(3atan(2724962))+619cos(3atan(2724962))−933+924962i19
/ / 1 \ / / 1 \ \ \ / / 1 \ / / 1 \ \ \
| 19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\| | 19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\|
| | __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 ||| | | __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 ||| / __________________\
| | / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------||| | | / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------||| | / ______ |
| | / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /|| | | / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /|| | / 2*I*\/ 4962 |
| |3 / 3 + ------------ *|- - - -------|| | |3 / 3 + ------------ *|- - - -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------|| | |3 / 3 + ------------ *|- - + -------|| | |3 / 3 + ------------ *|- - + -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------|| | 3 / 3 + ------------ |
| \\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 /| | \\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 /| | 19 \/ 9 |
|- ---------------------------------------------- + I*|- ---------------------------------------------- + ---------------------------- + ----------------------------| - ---------------------------- + ----------------------------|*|- ---------------------------------------------- + I*|- ---------------------------------------------- - ---------------------------- + ----------------------------| + ---------------------------- + ----------------------------|*|- ------------------------- - -----------------------|
\ 9 \ 9 6 18 / 6 18 / \ 9 \ 9 6 18 / 6 18 / | __________________ 3 |
| / ______ |
| / 2*I*\/ 4962 |
| 9*3 / 3 + ------------ |
\ \/ 9 /
−619sin(3atan(2724962))−919re((−21−23i)33+924962i1)+1857cos(3atan(2724962))+i−919im((−21−23i)33+924962i1)+1857sin(3atan(2724962))+619cos(3atan(2724962))619sin(3atan(2724962))+1857cos(3atan(2724962))−919re((−21+23i)33+924962i1)+i−619cos(3atan(2724962))+1857sin(3atan(2724962))−919im((−21+23i)33+924962i1)−333+924962i−933+924962i19
/ / / ______\\\
| | |2*\/ 4962 |||
| |2*atan|----------||| / 2/3 \
3 ___ | | \ 27 /|| |/ ______\ 3 ___|
19*\/ 3 *|1 - 2*cos|------------------||*\\27 + 2*I*\/ 4962 / + 19*\/ 3 /
\ \ 3 //
----------------------------------------------------------------------------
___________________
3 / ______
243*\/ 27 + 2*I*\/ 4962
243327+24962i1933(1−2cos(32atan(2724962)))(1933+(27+24962i)32)
19*3^(1/3)*(1 - 2*cos(2*atan(2*sqrt(4962)/27)/3))*((27 + 2*i*sqrt(4962))^(2/3) + 19*3^(1/3))/(243*(27 + 2*i*sqrt(4962))^(1/3))
/ 1 \ / / 1 \ \
19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\
| __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 ||
| / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------||
| / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /|
|3 / 3 + ------------ *|- - - -------|| | |3 / 3 + ------------ *|- - - -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------|
\\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 /
x1 = - ---------------------------------------------- + I*|- ---------------------------------------------- + ---------------------------- + ----------------------------| - ---------------------------- + ----------------------------
9 \ 9 6 18 / 6 18
x1=−619sin(3atan(2724962))−919re((−21−23i)33+924962i1)+1857cos(3atan(2724962))+i−919im((−21−23i)33+924962i1)+1857sin(3atan(2724962))+619cos(3atan(2724962))
/ 1 \ / / 1 \ \
19*re|---------------------------------------| | 19*im|---------------------------------------| / / ______\\ / / ______\\| / / ______\\ / / ______\\
| __________________ | | | __________________ | | |2*\/ 4962 || | |2*\/ 4962 ||| | |2*\/ 4962 || | |2*\/ 4962 ||
| / ______ / ___\| | | / ______ / ___\| |atan|----------|| |atan|----------||| |atan|----------|| |atan|----------||
| / 2*I*\/ 4962 | 1 I*\/ 3 || | | / 2*I*\/ 4962 | 1 I*\/ 3 || ____ | \ 27 /| ____ | \ 27 /|| ____ | \ 27 /| ____ | \ 27 /|
|3 / 3 + ------------ *|- - + -------|| | |3 / 3 + ------------ *|- - + -------|| \/ 19 *cos|----------------| \/ 57 *sin|----------------|| \/ 19 *sin|----------------| \/ 57 *cos|----------------|
\\/ 9 \ 2 2 // | \\/ 9 \ 2 2 // \ 3 / \ 3 /| \ 3 / \ 3 /
x2 = - ---------------------------------------------- + I*|- ---------------------------------------------- - ---------------------------- + ----------------------------| + ---------------------------- + ----------------------------
9 \ 9 6 18 / 6 18
x2=619sin(3atan(2724962))+1857cos(3atan(2724962))−919re((−21+23i)33+924962i1)+i−619cos(3atan(2724962))+1857sin(3atan(2724962))−919im((−21+23i)33+924962i1)
__________________
/ ______
/ 2*I*\/ 4962
3 / 3 + ------------
19 \/ 9
x3 = - ------------------------- - -----------------------
__________________ 3
/ ______
/ 2*I*\/ 4962
9*3 / 3 + ------------
\/ 9
x3=−333+924962i−933+924962i19
x3 = -(3 + 2*sqrt(4962)*i/9)^(1/3)/3 - 19/(9*(3 + 2*sqrt(4962)*i/9)^(1/3))