[BUG]多元多项式的乘法
>> :mode polyn
Switch into polynomial mode.
>> symbol(x,y)
out> x is a variable for polynomial.
out> y is a variable for polynomial.
------------------------
>> (x+y)(x-y)
in> (x+y)*(x-y)
out> +1x^2-1xy+1xy-1y^2
------------------------
>> (x+y)(x-y)+0
in> (x+y)*(x-y)+0
out> +1x^2+0-1y^2+0
------------------------
>> (x^2+xy+y^2)(x-y)
in> (x^2+xy+y^2)*(x-y)
out> +1x^3-1x^2y+1x^2y+0x^y^-1y^3
------------------------
>> (x^2+xy+y^2)(x-y)+0
in> (x^2+xy+y^2)*(x-y)+0
out> +1x^3+0-1y^3
------------------------
>>
海伦公式 (见(1 封私信) 怎么优雅的计算出这个海伦公式？ - 知乎 (zhihu.com))
>> symbol(a,b,c)
out> a is a variable for polynomial.
out> b is a variable for polynomial.
out> c is a variable for polynomial.
------------------------
>> (a+b+c)(a+b-c)(a-b+c)(-a+b+c)
in> (a+b+c)*(a+b-c)*(a-b+c)*(-a+b+c)
out> -a^4+2a^2b^2+0a^b^c^+2a^2c^2+0a^b^+0a^b^c^+0a^b^c^+0a^b^c^+0a^b^c^+0a^b^c^-b^4+2b^2c^2+0a^c^+0b^c^-c^4
------------------------
>>
只需测试
>> ab+ca-ac
v608已经解决.