I assume you're using sprintf because it was in the example code you copied. As mentioned above, you really don't need it:
float sensor1 = beta /(log((1025.0 * 10 / a - 10) / 10) + beta / 298.0) - 273.0;
float c = bmp280.getPressure();
Serial.print(Sensor1);
Serial.print(", ");
Serial.println(c);
Not tested. Not even compiled.