[tex](a\pm b)^2=a^2\pm2ab+b^2\\\\1.\ x^2-16x+c=x^2-2x\cdot8+c\to c=8^2=64\\\\x^2-16x+64=x^2-2x\cdot8+8^2=(x-8)^2\\\\\\2.\ p^2-14p+c=p^2-2p\cdot7+c\to c=7^2=49\\\\p^2-14p+49=p^2-2p\cdot7+7^2=(p-7)^2[/tex]
[tex]3.\ b^2+18b+c=b^2+2b\cdot9+c\to c=9^2=81\\\\b^2+18b+81=b^2+2b\cdot9+9^2=(b+9)^2\\\\\\4.\ n^2-n+c=n^2-2n\cdot\frac{1}{2}\to c=(\frac{1}{2})^2=\frac{1}{4}\\\\n^2-n+\frac{1}{4}=n^2-2n\cdot\frac{1}{2}+(\frac{1}{2})^2=(n-\frac{1}{2})^2[/tex]