已知R1=28<img data-fr-image-pasted="true" width="17" height="17" src="data:image/gif;base64,R0lGODlhEQARAHcAMSH+GlNvZnR3YXJlOiBNaWNyb3NvZnQgT2ZmaWNlACH5BAEAAAAALAIAAwANAAsAhAAAAAAAAAMAAA0KCAQIDQoEAAAECg0IBAADCAsLDQMGCw0LCwgDAAAABAoLDQAAAwgKDQ0LCgQAAAsGAwgIDQQAAwQGCwECAwECAwECAwECAwECAwECAwECAwECAwECAwVCICAOQmAGhKgWgaEeAZKIsKLSLbAwjXPrjAckIpH9ABMU0fhLEnbM20QIOFBvOxetdytoBxRApAIBpgYWkom6OylCADs=" class="fr-fic fr-dii"> R2=30<img data-fr-image-pasted="true" width="17" height="17" src="data:image/gif;base64,R0lGODlhEQARAHcAMSH+GlNvZnR3YXJlOiBNaWNyb3NvZnQgT2ZmaWNlACH5BAEAAAAALAIAAwANAAsAhAAAAAAAAAMAAA0KCAQIDQoEAAAECg0IBAADCAsLDQMGCw0LCwgDAAAABAoLDQAAAwgKDQ0LCgQAAAsGAwgIDQQAAwQGCwECAwECAwECAwECAwECAwECAwECAwECAwECAwVCICAOQmAGhKgWgaEeAZKIsKLSLbAwjXPrjAckIpH9ABMU0fhLEnbM20QIOFBvOxetdytoBxRApAIBpgYWkom6OylCADs=" class="fr-fic fr-dii">，如果把这两个电阻串联起来，其总电阻为(　　)。