题目

She is widely seen as proof that good looks can last for ever. But, at nearly 500 years of age, time is catching up with the Mona Lisa. The health of the famous picture, painted by Leonardo da Vinci in 1505, is getting worse by the year, according to the Louver Museum where it is housed. “The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape since experts checked it two years ago,” the museum said. Visitors have noticed changes but repairing the world’s most famous painting is not easy. Experts are not sure about the materials the Italian artist used a先化简，再求代数式的值，其中a=3tan30°+1，b=cos45°．