What number is halfway between 8.20 and 8.30

The difference between tax advoidance and tax evasion

If the speed of an electromagnetic wave is 100,000 cm/s and its wavelength is 100 cm, what is its frequency? a) 100,000 cycles per second (hz) b) 1 × 10-3 cycles per second (hz) c) 10,000,000 cycles per second (hz) d) 1,000 cycles per second (hz)

Can someone me with #s 8, 9, and 11. with just one of the three also works. prove using only trig identities.