设另一个直角边是x,则斜边长是12-4-x = 8-x
由勾股定理得
4*4 +x*x =(8-x)*(8-x)
即:16+X*X = 64-16X+X*X
16X = 48
X=3
面积=两直角边乘积/2=3*4/2 = 6
(斜边长5)