f(sinα+cosα)=sinαcosα
f(cos30`)
f(cos15`-sin15`)f(cos15+sin15)
=-sin15cos15sin15cos15
=-sin15^2cos15^2
=-[(1-cos30)/2][(1+cos30)/2]
=-[(1-cos30^2)/2]
=-[(1-3/4)/2]
=-1/8