cos(π/4 -b) = cos [ π/2 - (π/4+b) ] = sin(π/4 +b)诱导公式：cos(π/2 - x) = sinx如果你要化掉π/4就要用两角差的余弦公式cos(x-y) = cosxcosy - sinxsiny∴ cos(π/4 -b) = cos(π/4)cosb - sin(π/4)sinb= ...