0＜α＜π/4
则π/4＜α+π/4＜π/2
∴cos(α + π/4)＞0
∴cos(α + π/4)=4/5
cosα
=cos[(α + π/4) - π/4]
=cos(α + π/4)cosπ/4 + sin(α + π/4)sinπ/4
=(4/5)×(√2/2) + (3/5)×(√2/2)
=(7/10)√2