Question

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Answer 1

$1.\left ( -\frac{a}{7}-0,4b \right )\left ( 0,4b-\frac{a}{7} \right )=\left ( -\frac{a}{7}-0,4b \right )\left ( -\frac{a}{7}+0,4b \right )=\frac{a^2}{49}-\frac{4b^2}{25}\\\\2.(2n+3)^2-(2n+1)^2\equiv 0(mod8)\Leftrightarrow (2n-2n+3-1)(2n+2n+3+1)\equiv 0(mod8)\Leftrightarrow \\\Leftrightarrow 8(n+1)\equiv 0(mod8)\blacksquare \\3.397,5\cdot 402,5=\frac{795}{2}\cdot \frac{805}{2}=\frac{(800-5)(800+5)}{4}=\frac{800^2-25}{4}=\frac{639975}{4}$

Answer 2

Ответ:

Объяснение:

2.88$(-\frac{a}{7}-0,4b)(0,4b -\frac{a}{7})=-(0,4b+\frac{a}{7})(0,4b -\frac{a}{7})=-((0,4b)^{2}- (\frac{a}{7} )^{2} )=-0,16b^{2} +\frac{a^{2} }{49}$

2.89$(2n+3)^{2} -(2n+1)^{2}= 4n^{2} +12n+9 - (4n^{2} +4n+1)=4n^{2}+12n+9-4n^{2}-4n-1 =8n+8 = 8(n+1)$ кратно 8

2.90$397,5*402,5=(400-2,5)(400+2,5)=400^{2} -2,5^{2} =160 000-6,25 = 159 993,75$