五點(diǎn)法的關(guān)鍵是要列一個(gè)三行的表,中間一行的取值是0,π/2,π,3π/2,2π,這的固定的.
x              0       π/6       π/3       π/2        2π/3          
3x            0       π/2         π       3π/2        2π          
y            1/2       0        -1/2         0           1/2
當(dāng)　3x  分別取0,π/2,π,3π/2,2π時(shí),x的值分別是 0,π/6 ,π/3 ,π/2 , 2π/3 ,對(duì)應(yīng)的y值分別是 1/2, 0  , -1/2 , 0  ,1/2.這樣,根據(jù)第一行和第三行,就可以畫出y=(1/2)cos3x 在一個(gè)周期的圖像.