由已知得
f(a)=[1+√ 2cos(2a-π/4)]/sin(a+π/2)
=(1+cos2a+sin2a)/cosa
=(2cos²a+2sinacosa)/cosa
=2cosa+2sina
∵角a在第四象限且cosa=3/5
∴sina=-4/5
∴f(a)=6/5-8/5=-2/5