sin(a—π/4)=√2/10又a∈(0，π/2)，所以a-π/4∈(-π/4，π/4)cos(a-π/4)=7√2/10sin a=sin[(a-π/4)+π/4]=sin(a-π/4)cosπ/4+cos(a-π/4)sinπ/4=√2/10*√2/2+7√2/10*√2/2=4/5cos a=3/5tan a=4/3tan2a=2tan a/1-tana =-24/7
