题目

已知tan，tanαtanβ=，求cos（α－β）的值． 答案：解：∵tanαtanβ=， ∴cos（α－β）=－cos（α+β）． 又tan，∴cos（α+β）=， 从而cos（α－β）=－×（－）=．12．71．Susan，a heavy girl，eats much meat and few vegetables every day．C72．Mike doesn't like drinking milk or fruit juice．He only drinks a little water each day．A73．Paul always has sweet cake for breakfast or dinner．But he only brushes his teeth three or four times each week．D74．Lisa is a quiet girl．In her free time，she always reads or plays computer games at home．E75．Larry usually gets up late in the morning．So she often rushes to school and seldom eats breakfast．BHealthy tips from Dr．JacksonA．Do you know that more than half of your weight is water？To keep healthy，  you need at least 8 glasses of water a day．B．Our bodies need energy to work．So，before you leave for school，start the day      with breakfast．It can give you energy．C．You need over 40 different nutrients every day for health．You can get   enough nutrients if you eat different foods like vegetables，fruit and meat．D．Care for you teeth!Brush your teeth at least twice a day．Also，after brushing    your teeth at bedtime，don't eat any fo od or drink anything but water．E．Our body needs to keep moving．Outdoor activities can help your body healthy and strong．