a∈（π/2，π）a-π/4∈（π/4，3π/4）sin(a-π/4)=3/5cosa=cos[(a-π/4)+π/4]=cos(a-π/4)cosπ/4-sin(a-π/4)sinπ/4=4/5*√2/2-3/5*√2/2=4√2/10-3√2/10=√2/10
