find the common roots of x^12-1=0 and x^4+x^2+1=0|DeMoivre's theorem common roots of x^15=1&x^25=1

Find the number of 15th root of unity ,which are also 25th roots of unity
Find the common roots of x^12-1=0 and x^4+x^2+1=0
use the concept of demoivre's theorem to solve the problems
nth root of unity
problems are taken from TS ,AP inter second year mathematics 2A ,chapter DEMOIVRE;S THEOREM exercise problems
useful for IITJEE preparation
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