[填空题]
In the design of a supermarket, the7xm5tgf j+0x2xd 6osq g+ fvf:+ i+cl*.9if p6 iymd.b6oyma yre are to be several ramps connecting different parts of the std f9mm6y p6y..*i ++bycl iaioore.
Customers will have to push grocery carts up the ramps and it is obviously desirable that this not be too difficult. The engineer has done a survey and found that almost no one complains if the force directed up the ramp is no more than 20 N . Ignoring friction, at what maximum angle $\theta$ should the ramps be built, assuming a full $30-\mathrm{kg}$ grocery cart? $^{\circ} $
