观察下列等式：①sin²10°＋cos²40＋sin10°cos40°＝3／4；②sin²

是sin²a+cos²(a+30)+sinacos(a+30)=3/4
左边=sin²a+(cosacos30-sinasin30)²+sina(cosacos30-sinasin30)
=sin²a+3/4*cos²a-√3/2*sinacosa+1/4*sin²a+√3/2*sinacosa-1/2*sin²a
=3/4*sin²a+3/4*cos²a
=3/4(sin²a+cos²a)
=3/4=右边