Test analysis: (1) The initial mass is 500g,

1 year later, ω = 500 (1- 10) = 500×,

Two years later, ω=500×,

……,

It is concluded that after t years, ω = 500×. -Five points.

(2) Solve equation 500×250.

= ,

,

,

Therefore, the half-life of this radioactive element is about 20 years.-10 minute.

Comments: the first question of this question summarizes the residual content after t years from the residual quality after one year and two years. The second problem involves the conversion from exponent to logarithm.