answer:To find the exact values: 1. ( cos 270^o ) Since ( 270^{circ} ) is in the third quadrant, where cosine is negative, and ( cos theta = -sin (90^{circ} - theta) ), we can write: [ cos 270^{circ} = -sin (90^{circ} - 270^{circ}) = -sin (-180^{circ}) ] Now, ( sin (-180^{circ}) = -sin 180^{circ} ) and ( sin 180^{circ} = 0 ), so: [ cos 270^{circ} = 0 ] [ color{blue}{boxed{cos 270^{circ} = 0}} ] 2. ( sin 180^{circ} ) Using the angle addition formula ( sin (180^{circ} - theta) = sin theta ), we have: [ sin 180^{circ} = sin (180^{circ} - 0^{circ}) = sin 0^{circ} ] Since ( sin 0^{circ} = 0 ), we get: [ sin 180^{circ} = 0 ] [ color{blue}{boxed{sin 180^{circ} = 0}} ] Thus, the exact values are ( cos 270^o = 0 ) and ( sin 180^0 = 0 ).

answer:Macrophages are target cells for HIV and play a role in the spread of the virus. Dendritic cells are antigen-presenting cells that activate T cells, which are crucial for the cellular immune response against HIV.

answer:The expression b^2 - 4ac is known as the discriminant. Its value determines the nature of the solutions to the quadratic equation: * If b^2 - 4ac > 0, the equation has two distinct real solutions. * If b^2 - 4ac = 0, the equation has two equal real solutions (a double root). * If b^2 - 4ac < 0, the equation has two complex conjugate solutions.

answer:The quadratic formula provides a set of instructions to find the roots (solutions) of a quadratic equation. It is given by: {eq}x = frac{-b pm sqrt{b^2 - 4ac}}{2a} {/eq} Here, a, b, and c are the coefficients of the quadratic equation ax^2 + bx + c = 0, and the symbol "±" indicates that there are two possible roots, one calculated with the positive sign and the other with the negative sign. The expression under the square root, (b^2 - 4ac), is known as the discriminant, which helps determine the nature of the roots (real and distinct, equal, or complex).