方法1:因为((sin^2)x+(cos^2)x)^2=(sin^4)x+(cos^4)x+2*(sin^2)x(cos^2)x
由(sinx+cosx)^2=(sin^2)x+(cos^2)x+2*sinxcosx,即
sinxcosx=((sinx+cosx)^2-=(sin^2)x+(cos^2)x)/2=(2-1)/2=1/2
所以(sin^4)x+(cos^4)x=((sin^2)x+(cos^2)x)^2-2*(sin^2)x(cos^2)x=1-2*(1/2)^2=1/2
方法2:取特值取sinx=cosx=√2/2,即(sin^4)x+(cos^4)x=(√2/2)^4+(√2/2)^4=1/2