1.向量AB*CA=15/4
∴|AB*|CA|*COS∠BAC=15/4
∴cos∠BAC=1/4∴∠BAC=arccos1/4
2.∵sin（π/4-x)=5/13
∴cosx-sinx=5/13*根号2∴1-2sinxcosx=50/169 ,2sinxcosx=119/169
∴（cosx+sinx)^2=1+2sinxcosx=288/169∵00∴cosx+sinx=12/13*根号2
∴原式=（cos²x-sin²x）/【根号2/2*(cosx-sinx)]
=根号2*（cosx+sinx)
=24/13