Graphic optimization problem (matlab syntax)

I'm trying to minimize z = 0,1h^2 +0,8ht so we have two variables t and h. In this assignment, we have few constant variables:

E = 205000

F = 4000

L = 1500

Mt = F*L

sigma = 180

vsall = 14

First two restriction equations are easy

1) t <= h/8

2) t >= h/20

These were pretty easy imo:

t=linspace(5,15)

h=linspace(50,200)

[x1,y1]=meshgrid(t,h)

(5,15 and 50,200 are from my head, and good for this assignment)

rajoitus1 = x1 - y1/20

rajoitus2 = x1 - y1/8

contour (x1,y1,rajoitus1,[0,0])

contour (x1,y1,rajoitus2,[0,0])

Rajoitus = restriction in Finnish! These draw correct straight lines for the restriction 1 and 2.

Restriction 3/4 are bit more complex. They consist a variable Iz = Iz(t,h) which is defined under.

Iz = (1/24)*y1^4 - 1/30*y1*(y1-2*x1)^3

3) vsall >= (F*L^3)/(3*E*Iz)

4) sigma >= (Mt*y1/2) / Iz

When I'm trying to do same with these two as I did for the 1) and 2):

rajoitus3 = vsall - (F*L^3)/(3*E*Iz)

rajoitus4 = sigma - (Mt*y1/2) / Iz

Error for the restriction 3:

""Error using / Matrix dimensions must agree.""

Warning for the restriction 4:

"Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND = 6.143622e-35. "

What am I doing wrong? What do I have to do if I want to get these four "restriction lines" to same picture (t,h coordinate system)