[填空题]
What would be the wavelengths of the two photons produdlltl.x v6i b :c i2f/z)r3k/aced when an electron74yrm oqenq:y+d b lu 8)8n1wivhmk.9;um h z, and a positron, each with 420 keV of kinetivh l :8q +iq.eymm ;ru7 4,8nowhbnmykz 1)u9dc energy, annihilate head on? $\lambda $ $\approx $ $ \times 10^{-12}\ \text{m}$
