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1) (3b-c)(3b+c)=9b²-c² - верно
2) (x+4)(4-x)=16-x² - верно
3) (6n+7)(7-6n)=36n²-49 не верно, будет 49-36n²
4) (y²-5)(y²+5)=y^4-25 - верно
3t^2 - 2t - 1 = 0
D = 4 + 12 = 16 ; V D = 4
t1 = ( 2 + 4 ) : 6 = 1
t2 = ( 2 - 4 ) : 6 = - 1/3
Ответ 1 ; - 1/3