DEF f(x)=x^2
DEF g(x)=2*x      !g(x)はf(x)の微分
DEF h(x)=g(i)*x+(f(i)-g(i)*i)    !f(x)=x^2 の接線たち：傾きg(i)で点（i,f(i)）を通る直線
SET WINDOW -2,2,-2,2
FOR i=-2 TO 2 STEP 0.005
   FOR j=-2 TO 2 STEP 0.005
      PLOT LINES : j,h(j)
   NEXT j
NEXT i
END