a)
(2+x)^2=2^2+2*2*x+x^2=4+4x+x^2
b)
(3x-4y )^2=(3x)^2-2*3x*4y+(4y)^2=9x^2-24xy+16y^2
c)
(11+13y) (11-13y)=11^2-(13y)^2=121-169y^2
d).
19^2=(20-1)^2=20^2-2*20*1+1=400-4)+1=361
e)
61^2=(60+1)^2=60^2+2*60*1+1=3600+121=3721
f)
17*23=(20-3)(20+3)=20^2-3^2=400-9=391