We have \(y_1=0+0.5\cdot 1^2=0.5\), because the slope is evaluated at \(t_0=1\). The value \(0.625\) uses \(t_1=1.5\) in the slope, while \(0.25\) omits the factor \(h\) and \(1.125\) mixes both errors.
Euler's method approximates the solution of an initial value problem \(y'=f(t,y)\), \(y(t_0)=y_0\), advancing in steps of size \(h\) using the slope at the current point.