a、b为锐角
sina=tana/√(1+tan^2a),cosa=1/√(1+tan^2a)
sinb=tanb/√(1+tan^2b),cosb=a/√(1+tan^2b)
sin(a+b)=sinacosb+cosasinb
=(tana+tanb)/√(1+tan^2a)(1+tan^2b)
=(2+3)/√(1+4)(1+9)
=5/5√2=√2/2