36×7+28=252+28=280
................
3+7,8=3,0+7,8=10,8 изи же
Х руб.-100%
60 руб.-20%
20х=60*100
20х=6000
х=6000:20
х=300(руб.)-первоначально.
1) x^2 + 4x<span> - 21 = 0
D = </span>4^2<span> - 4·1·(-21) = 16 + 84 = 100
</span>x1 = (<span><span>-4 - √100)/</span>2·1</span> = (<span>-4 - 10)/2</span> = <span>-14/2</span><span> = -7
</span>x2 = (<span><span>-4 + √100)/</span>2·1</span> = (<span>-4 + 10)/2</span> = <span>6/2</span><span> = 3
</span>2) 2x^2 + 10x<span> = 0
</span>D = 10^2<span> - 4·2·0 = 100 - 0 = 100
</span>x1 = (<span><span>-10 - √100)/</span>2·2</span> = (<span>-10 - 10)/4</span> = <span>-20/4</span><span> = -5
</span>x2 = (<span><span>-10 + √100)/</span>2·2</span> = (<span>-10 + 10)/4</span> = <span>0/4</span><span> = 0
</span>3) x^2 + 3x<span> - 18 = 0
</span>D = 3^2<span> - 4·1·(-18) = 9 + 72 = 81
</span>x1 = (<span><span>-3 - √81)/</span>2·1</span> = (<span>-3 - 9)/2</span> = <span>-12/2</span><span> = -6
</span>x2 = (<span><span>-3 + √81)/</span>2·1</span> = (<span>-3 + 9)/2</span> = <span>6/2</span><span> = 3
</span>4) 5x^2 + 15x<span> = 0
</span>D = 15^2<span> - 4·5·0 = 225 - 0 = 225
</span>x1 = (<span><span>-15 - √225)/</span>2·5</span> = (<span>-15 - 15)/10</span> = <span>-30/10</span><span> = -3
</span>x2 = (<span><span>-15 + √225)/</span>2·5</span> = (<span>-15 + 15)/10</span> = <span>0/10</span><span> = 0</span>
<span>5) x = -20
</span>
Два+два будет четыре,
2+2=4