有两个电阻R1和R2的阻值之比为2:3,串联后接在电源上,则R1和R2的电流之比I1:I2=______,电压之比U1:U2=______;将它们并联后接在电源上,则R1和R2的电流之比I1′:I2′=______,电压之比U1′:U2′=______.