y=2x+2

(2) A(0，2)，B(- 1，0)

M: y=2x+t, then c (-t/2,0)

BC = |- 1-(-t/2)| = | t/2- 1 |

OA=2

s = BC * OA/2 = | t/2- 1 | * 2/2 = | t/2- 1 |

When t/2- 1 >: 0, t>2, S=t/2- 1.

When t=2, s = 0.

At t<2, S= 1-t/2.