魏笑的回答：因为(sinx)^4=((sinx)^2)^2,cos2x=1-2(sinx)^2 所以(sinx)^2=(1-cos2x)/2 所以(sinx)^4=((1-cos2x)/2)^2=(1-2cos2x+(cos2x)^2)/4 又因为(cos2x)^2=(cos4x+1)/2 所以(sinx)^4=(cos4x-4cos2x+3)/8 结束。的回答：f(x)=4cosx(sinx+cosx) =4sinxcosx+4cos2x-2+2 =2sin2x+2cos2x+2 =(2√2)sin(2x+(π/4))+2 |
