题目

Malaria, the world's most widespread parasitic（寄生虫引起的） disease, kills as many as three million people every year—almost all of whom are under five, very poor, and African．In most years, more than five hundred million cases of illness result from the disease, although exact numbers are difficult to assess because many people don't （or can't） seek care．It is not unusual for a family earning less than two hundred dollars a year to spend a quarter of its income on malaria treatment, and what they often get no longer works．In countries like Tanzania, Mozambique, and the 计算tan45°-cos60°cos30°•sin60°=______．