都是直接套公式的
1.0.49a^2-144b^2
=(0.7a+12b)(0.7a-12b)
2.25/4a^2-x^2y^2
=(5/2a+xy)(5/2a-xy)
3.(m+n)^2-n^2
=(m+n+n)(m+n-n)
=m(m+2n)
4.169(a-b)^2-196(a+b)^2
=[13(a-b)+14(a+b)][13(a-b)-14(a+b)]
=(27a+b)(-a-27b)
=-(27a+b)(a+27b)
5.b^2-22b+121
=(b-11)^2
6.y^2+y+1/4
=(y+1/2)^2