∵∠C=90°,
∴c^2=a^2+b^2,
∵a:c=3:5,设 a=3X(X>0),c=5X,
根据勾股定理得:
25X^2=9X^2+16^2,
16X^2=256
X^2=16,
X=4,
∴a=12,c=20.