The teacher will be very surprised to see your correct solution 😉
/ 2 \ / 2 \ / 2 \ \x - 3*x + 2/*\x - 9*x + 20/*\x - 9*x + 18/ = 720
x1 = 0
x2 = 7
/ / ____\\ / / ____\\
| |8*\/ 14 || | |8*\/ 14 ||
|atan|--------|| |atan|--------||
3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|
3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|
7 \ 2 / \ 2 /
x3 = - - ------------------------------- + ---------------------------------
2 2 2
/ / ____\\ / / ____\\
| |8*\/ 14 || | |8*\/ 14 ||
|atan|--------|| |atan|--------||
3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|
3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|
7 \ 2 / \ 2 /
x4 = - + ------------------------------- - ---------------------------------
2 2 2
/ / ____\\ / / ____\\
| |8*\/ 14 || | |8*\/ 14 ||
|atan|--------|| |atan|--------||
3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|
3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|
7 \ 2 / \ 2 /
x5 = - - ------------------------------- - ---------------------------------
2 2 2
/ / ____\\ / / ____\\
| |8*\/ 14 || | |8*\/ 14 ||
|atan|--------|| |atan|--------||
3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|
3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|
7 \ 2 / \ 2 /
x6 = - + ------------------------------- + ---------------------------------
2 2 2
x6 = 3^(3/4)*35^(1/4)*sin(atan(8*sqrt(14)/7)/2)/2 + 7/2 + 3^(3/4)*35^(1/4)*i*cos(atan(8*sqrt(14)/7)/2)/2
sum
/ / ____\\ / / ____\\ / / ____\\ / / ____\\ / / ____\\ / / ____\\ / / ____\\ / / ____\\
| |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 || | |8*\/ 14 ||
|atan|--------|| |atan|--------|| |atan|--------|| |atan|--------|| |atan|--------|| |atan|--------|| |atan|--------|| |atan|--------||
3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|
3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------| 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------| 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------| 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|
7 \ 2 / \ 2 / 7 \ 2 / \ 2 / 7 \ 2 / \ 2 / 7 \ 2 / \ 2 /
7 + - - ------------------------------- + --------------------------------- + - + ------------------------------- - --------------------------------- + - - ------------------------------- - --------------------------------- + - + ------------------------------- + ---------------------------------
2 2 2 2 2 2 2 2 2 2 2 2
=
21
product
/ / / ____\\ / / ____\\\ / / / ____\\ / / ____\\\ / / / ____\\ / / ____\\\ / / / ____\\ / / ____\\\
| | |8*\/ 14 || | |8*\/ 14 ||| | | |8*\/ 14 || | |8*\/ 14 ||| | | |8*\/ 14 || | |8*\/ 14 ||| | | |8*\/ 14 || | |8*\/ 14 |||
| |atan|--------|| |atan|--------||| | |atan|--------|| |atan|--------||| | |atan|--------|| |atan|--------||| | |atan|--------|| |atan|--------|||
| 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|| | 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|| | 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /|| | 3/4 4 ____ | \ 7 /| 3/4 4 ____ | \ 7 /||
| 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|| | 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|| | 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------|| | 3 *\/ 35 *sin|--------------| I*3 *\/ 35 *cos|--------------||
|7 \ 2 / \ 2 /| |7 \ 2 / \ 2 /| |7 \ 2 / \ 2 /| |7 \ 2 / \ 2 /|
0*7*|- - ------------------------------- + ---------------------------------|*|- + ------------------------------- - ---------------------------------|*|- - ------------------------------- - ---------------------------------|*|- + ------------------------------- + ---------------------------------|
\2 2 2 / \2 2 2 / \2 2 2 / \2 2 2 /
=
0
0
x1 = 5.22267424002185 - 2.17200518812337*i
x2 = 1.77732575997815 + 2.17200518812337*i
x3 = 5.22267424002185 + 2.17200518812337*i
x4 = 0.0
x5 = 1.77732575997815 - 2.17200518812337*i
x6 = 7.0
x6 = 7.0