题目

已知lga,lgb,lgc成等差数列,且d＞0,a、b、c分别是Rt△ABC中∠A、∠B、∠C的对边,则sinA的值是(    )A.            B.            C.不存在             D. 答案：A解析:由2lgb=lga+lgc有b2=ac.在Rt△ABC中,由公差d＞0,∴lga＜lgb＜lgc,从而a＜b＜c,∴b2=c2-a2,则c2-a2=ac.∴两边同除以c2得1-sin2A=sinA,得sinA=,取sinA＞0.What are hurricanes? You may already know that hurricanes are storms. They can causeterrible waves, winds, and rains. They happen during “Hurricane Season,” which is from June 1st until November 30th in the Atlantic Ocean and from May 15th until November 30th in the Pacific Ocean. Kinds of Hurricanes There are five kinds of hurricanes. it helps people know a hurricane is so bad because the greater the wind speed is, the more dangerous the storm is. For example, a kind of hurricane has winds over 155 miles per hour, which are strong enough to break the buildings along the seaside completely. Hurricane Dangers When a hurricane happens together with heavy rain, there can be a terrible flood. The centre of a hurricane is called the eye. While most of a hurricane has dangerous strong winds, when the eye is a calm area in the storm in fact. When the eye of a hurricane passes over land, people might think that it's over, but before long the wind and rain increase again as the second part of the hurricane moves through. Hurricane Safety Tips There is no way to stop a hurricane or make it change direction, so if you ever find yourself in the path of a hurricane, be sure to remember the following safety tips. ● Be sure you have a radio, fresh drinking water, and enough food. ● Tell neighbors, friends, and family members your emergency plans. Tell them where you'll go if you need to leave your home. ● Stay inside during the storm. You could be badly hurt if you go outside.HurricanesCharacteristics● Hurricane Season usually has about 【1】 months in the Atlantic Ocean and six and a half months in the Pacific Ocean. ● Terrible waves, wind, and rain are usually the 【2】 of hurricanes. 【3】 ● There are five kinds of hurricanes. Different hurricanes have different wind speeds.● A kind of hurricane can break all the buildings along the seaside.Dangers● When a hurricane comes together with heavy rain, a terrible flood may happen. ● The eye is the center of a hurricane. Soon after it passes, the wind and rain increase again.Safety Tips● It's 【4】 to stop a hurricane or change its direction.● Don't 【5】 the safety tips if you find yourself in the path of a hurricane. ● Stay inside during the storm, or you could be seriously hurt.