∫f(x,y)dxdy=∫cxydxdy=c∫xdx∫ydy
=c(1/2*x^2|从0到2)(1/2*y^2|从0到1)
=c(1/2*2^2-0)(1/2*1^2-0)
=c*2*1/2=c
并且∫f(x,y)dxdy=1
所以c=1