怎么用Matlab完成以下的几个问题？

第一题送分的

syms a b c d x
f = a*x^3 + b*x^2 + c*x +d

第二题

>> syms a b x y t
>> A = [a*cos(x), 10+20; a*x^2+b*y^2, sqrt(t^2+1)];
>> B = [sin(x), cos(y); -cos(y), sin(x)];
>> A+B
 
ans =
 
[      sin(x) + a*cos(x),              cos(y) + 30]
[ a*x^2 - cos(y) + b*y^2, sin(x) + (t^2 + 1)^(1/2)]
 
>> A-B
 
ans =
 
[      a*cos(x) - sin(x),              30 - cos(y)]
[ cos(y) + a*x^2 + b*y^2, (t^2 + 1)^(1/2) - sin(x)]
 
>> A*B
 
ans =
 
[                     a*cos(x)*sin(x) - 30*cos(y),                     30*sin(x) + a*cos(x)*cos(y)]
[ sin(x)*(a*x^2 + b*y^2) - cos(y)*(t^2 + 1)^(1/2), cos(y)*(a*x^2 + b*y^2) + sin(x)*(t^2 + 1)^(1/2)]
 
>> A/B
 
ans =
 
[                          (30*cos(y) + a*cos(x)*sin(x))/(cos(y)^2 + sin(x)^2),                           (30*sin(x) - a*cos(x)*cos(y))/(cos(y)^2 + sin(x)^2)]
[ (cos(y)*(t^2 + 1)^(1/2) + a*x^2*sin(x) + b*y^2*sin(x))/(cos(y)^2 + sin(x)^2), -(a*x^2*cos(y) - sin(x)*(t^2 + 1)^(1/2) + b*y^2*cos(y))/(cos(y)^2 + sin(x)^2)]
 
>> A.*B
 
ans =
 
[         a*cos(x)*sin(x),              30*cos(y)]
[ -cos(y)*(a*x^2 + b*y^2), sin(x)*(t^2 + 1)^(1/2)]

第三题：

syms x a
f = x^3+3*x^2-6*x+5;
subs(f, x, a)
subs(f, x, 5)

其它两题参考：
https://ask.csdn.net/questions/7571179?spm=1005.2025.3001.5141