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Đáp án + giải thích các bước giải :
$A=4x^2 - 7x+15\\⇒A=(2x)^2 - 2.2x . \dfrac{7}{4} + (\dfrac{7}{4})^2 +\dfrac{191}{16}\\⇒A=(2x-\dfrac{7}{4})^2+\dfrac{191}{16} \geqslant \dfrac{191}{16}>0∀x\\⇒A>0∀x\\C=a^2-ab+b^2\\⇒C=a^2-2.a.\dfrac{1}{2}b + (\dfrac{1}{2}b)^2 +\dfrac{3}{4}b^2\\⇒C=(a-\dfrac{1}{2}b)^2 +\dfrac{3}{4}b^2\geqslant 0∀a,b\\⇒C \geqslant ∀a,b\\E=9x^2 + 5y^2 - 6xy - 4y+7\\⇒E=(9x^2 - 6xy + y^2) + (4y^2 - 4y + 1) + 6\\⇒E=(3x-y)^2 + (2y-1)^2 + 6 \geqslant 6 > 0∀x,y\\⇒E > 0∀x,y$
