The heart rate of a person during exercise is usually related to his age. If a is used to indicate his age and B is used to indicate the maximum number of heart beats per minute that a person can bear during exercise, then B = 0.8 (220-a). (1) under normal circumstances, what is the maximum number of heart beats per minute that a 16-year-old teenager can bear during exercise? (2) When a person of 50 years old exercises, the frequency of 10 second heartbeat is 20 times. Is he in danger? Why?

(1) Substituting a = 16 into b = 163.2 times, a: the maximum number of heartbeat per minute that a 16-year-old can bear is 163.2 times; (2) substituting a = 50 into b = 136 times, 136 △ 60 × 10 = 683 > 20, so this person is not in danger

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