t = 20(0 & lt； = t < = 2) Because t keeps a constant temperature of 20℃ from 0: 00 to 2: 00.

t = 20+5(t-2)= 10+5t(2 & lt； = t & lt=4) Because the temperature rises at a constant speed from 2: 00 to 4: 00, it rises by 5℃ every hour.

When drawing, the ordinate is t and the abscissa is t.

Draw a straight line with T=20 at point 0-2, and then draw a straight line with T= 10+5t, and intersect with T=20 at point (20,2). Remember to erase the diagonal below T=20 or make a dotted line.