how can you use four rods that measure 2cm, 5cm, 7cm, and 9cm to measure a length of 1 cm?

Answer 1

( 5 + 7 ) - ( 9 + 2 ) = 12 - 11 = 1 cm

Put the 7cm and 5 cm rods together = 12 cm

Put the 9cm and 2cm rods together = 11 cm

Difference between is 1cm thus, you can use that difference to measure 1 cm.