Question #213425

sketch the gradient field for the differential equation dy/dt=t-y2 for t=-1......,1

y=-1,.......,1



1
Expert's answer
2021-07-19T18:38:24-0400

Let's calculate the values of the function f(t,y)=ty2f\left(t,y\right)=t-y^2 in the area A={(t,y)1t1,1y1}A=\left\{\left(t,y\right)\left|-1\le t\le1,\,\,\,-1\le y\le 1\right.\right\} .To be more detailed, let's take a step of Δt=Δy=0.5\Delta t=\Delta y=0.5 . Then,


t=1:{f(1,1)=1(1)2=2f(1,0.5)=1(0.5)2=1.25f(1,0)=102=1f(1,0.5)=10.52=1.25f(1,1)=112=2t=0.5:{f(0.5,1)=0.5(1)2=1.5f(0.5,0.5)=0.5(0.5)2=0.75f(0.5,0)=0.502=0.5f(0.5,0.5)=0.50.52=0.75f(0.5,1)=0.512=1.5t=0:{f(0,1)=0(1)2=1f(0,0.5)=0(0.5)2=0.25f(0,0)=002=0f(0,0.5)=00.52=0.25f(0,1)=012=1t=0.5:{f(0.5,1)=0.5(1)2=0.5f(0.5,0.5)=0.5(0.5)2=0.25f(0.5,0)=0.502=0.5f(0.5,0.5)=0.50.52=0.25f(0.5,1)=0.512=0.5t=1:{f(1,1)=1(1)2=0f(1,0.5)=1(0.5)2=0.75f(1,0)=102=1f(1,0.5)=10.52=0.75f(1,1)=112=0t=-1 : \left\{\begin{array}{l} f(-1,-1)=-1-\left(-1\right)^2=-2\\[0.3cm] f(-1,-0.5)=-1-\left(-0.5\right)^2=-1.25\\[0.3cm] f(-1,-0)=-1-0^2=-1\\[0.3cm] f(-1,0.5)=-1-0.5^2=-1.25\\[0.3cm] f(-1,1)=-1-1^2=-2 \end{array}\right.\\[0.3cm] t=-0.5 : \left\{\begin{array}{l} f(-0.5,-1)=-0.5-\left(-1\right)^2=-1.5\\[0.3cm] f(-0.5,-0.5)=-0.5-\left(-0.5\right)^2=-0.75\\[0.3cm] f(-0.5,-0)=-0.5-0^2=-0.5\\[0.3cm] f(-0.5,0.5)=-0.5-0.5^2=-0.75\\[0.3cm] f(-0.5,1)=-0.5-1^2=-1.5 \end{array}\right.\\[0.3cm] t=0 : \left\{\begin{array}{l} f(0,-1)=0-\left(-1\right)^2=-1\\[0.3cm] f(0,-0.5)=0-\left(-0.5\right)^2=-0.25\\[0.3cm] f(0,-0)=0-0^2=-0\\[0.3cm] f(0,0.5)=0-0.5^2=-0.25\\[0.3cm] f(0,1)=0-1^2=-1 \end{array}\right.\\[0.3cm] t=0.5 : \left\{\begin{array}{l} f(0.5,-1)=0.5-\left(-1\right)^2=-0.5\\[0.3cm] f(0.5,-0.5)=0.5-\left(-0.5\right)^2=0.25\\[0.3cm] f(0.5,-0)=0.5-0^2=0.5\\[0.3cm] f(0.5,0.5)=0.5-0.5^2=0.25\\[0.3cm] f(0.5,1)=0.5-1^2=-0.5 \end{array}\right.\\[0.3cm] t=1 : \left\{\begin{array}{l} f(1,-1)=1-\left(-1\right)^2=0\\[0.3cm] f(1,-0.5)=1-\left(-0.5\right)^2=0.75\\[0.3cm] f(1,-0)=1-0^2=1\\[0.3cm] f(1,0.5)=1-0.5^2=0.75\\[0.3cm] f(1,1)=1-1^2=0 \end{array}\right.

In the figure it looks like this


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