-4х в кв. + 15х + 4 =0
D = 225- (-64) = 289;
корень с D=17;
х1 = 4; х2 = -1\4 ;
sin 11 * cos19 + cos 11 * sin 19 = sin (11 + 19) = sin 30 = 1/2
Применена формула синуса суммы двух углов:
sin (A + B) = sinA*cosB + cosA*sinB,
и испоьзовано табличное значение синуса угла в 30 градусов
<span>a2 – b2 = (a + b)(a – b),</span>
<span>(a + b)2 = a2 + 2ab + b2,</span>
<span>(a – b)2 = a2 – 2ab + b2,</span>
<span>a3 + b3 = (a + b)(a2 – ab + b2),</span>
<span>a3 – b3 = (a – b)(a2 + ab + b2),</span>
<span><span>an – bn = (a – b)(a<span>n – 1</span> + a<span>n – 2</span>b + a<span>n – 3</span>b2 + … + ab<span>n – 2</span> + b<span>n – 1</span>)</span>,</span>
<span>(a + b)3 = a3 + 3a2b + 3ab2 + b3 = a3 + b3 + 3ab(a + b),</span>
<span>(a – b)3 = a3 – 3a2b + 3ab2 – b3 = a3 – b3 – 3ab(a – b).
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