已知tana=1/2求下列各式的值 sin^2a+sina*cosa+2

tan(a)=1/2sin²(a)+sin(a)*cos(a)+2=cos²(a)*[tan²(a)+tan(a)]+2=cos²(a)*(3/4)+2=1/[1/cos²(a)]*(3/4)+2=1/{[sin²(a)+cos²(a)]/cos²(a)}*(3/4)+2=1/[tan²(a)+1]*(3/4)+2=1/(1/4+1)*(3/4)+2=(4/5)*(3/4)+2=3/5+2=13/5