1)(x+2)(2-x)=4-х^2
2)(1-а)(1+а)=1-a^2
3)(n-2m)(n+2m)=n^2-4m^2
4)(2x-3y)(2x+3y)=4x^2-9y^2
х + 4у = 5
cos = 1/sqrt(1+tg^2)=1/sqrt5