1) с^2-0,49=с^2-0,7^2=(с-0,7)(с+0,7)
2) 16-k^2=4^2-k^2=(4-k)(4+k)
3) 400-m^2=20^2-m^2=(20-m)(20+m)
4) t^2-225=t^2-15^2=(t-15)(t+15)
5)1,69-b^2=1,3^2-b^2=(1,3-b)(1,3+b)
6) y^2-16/81=y^2-(4/9)^2=(y-4/9)(y+4/9)
7) 25x^2-4=(5x)^2-2^2=(5x-2)(5x+2)
8) 25/36-64y^2=(5/6)^2-(8y)^2=(5/6-8y)(5/6+8y)