電大開(kāi)放英語(yǔ)i(2)形考冊(cè)作業(yè)1-4參考答案小抄(自己整理)-wenkub.com
2025-06-01 19:11 本頁(yè)面
【正文】 a chord through the center is the diameter. A secant is a line of indefinite length intersecting the circle at two points, the segment of it within the circle being a chord. A tangent to a circle is a straight line touching the circle at only one point, the point of contact, or tangency, and is always perpendicular to the radius drawn to this point. A circle is inscribed in a polygon if each side of the polygon is tangent to the circle。 technically, the slope m of a straight line is the (trigonometric) tangent of the angle it makes with the xaxis. If the line is parallel to the xaxis, its slope is zero. Two or more lines with equal slopes are parallel to one another. In general, the slope of the line through the points (x1, y1) and (x2, y2) is given by m= (y2y1) / (x2x1). The conic sections are treated in analytic geometry as the curves corresponding to the general quadratic equation ax2+bxy+cy2+dx+ey+f=0, where a, b,?…?, f are constants and a, b, and c are not all zero. In solid analytic geometry the orientation of a straight line is given not by one slope but by its direction cosines, λ, μ, and ν, the cosines of the angles the line makes with the x, y, and zaxes, respectively。 the adoption of an approach to space in which all parts of the canvas played an equally vital role in the total work。s turbulent yet elegant abstract paintings, which were created by spattering paint on huge canvases placed on the floor, brought abstract expressionism before a hostile public. Willem de Kooning39。 and there are some of each type in the brain. These terms are used because nicotine mimics the action of acetylcholine at nicotinic receptors, whereas muscarine, an alkaloid from the mushroom Amanita muscaria, mimics the action of acetylcholine at the muscarinic receptors. Acetylcholine is the neurotransmitter produced by neurons referred to as cholinergic neurons. In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac musc le. In the central nervous system acetylcholine is believed to be involved in learning, memory, and mood. Acetylcholine is synthesized from choline and acetyl coenzyme A through the action of the enzyme choline acetyltransferase and bees packaged into membranebound vesicles . After the arrival of a nerve signal at the termination of an axon, the vesicles fuse with the cell membrane, causing the release of acetylcholine into the synaptic cleft . For the nerve signal to continue, acetylcholine must diffuse to another nearby neuron or muscle cell, where it will bind and activate a receptor protein. There are two main types of cholinergic receptors, nicotinic and muscarinic. Nicotinic receptors are located at synapses between two neurons and at synapses between neurons and skeletal muscle cells. Upon activation a nicotinic receptor acts as a channel for the movement of ions into and out of the neuron, directly resulting in depolarization of the neuron. Muscarinic receptors, located at the synapses of nerves with smooth or cardiac muscle, trigger a chain of chemical events referred to as signal transduction. For a cholinergic neuron to receive another impulse, acetylcholine must be released from the receptor to which it has bound. This will only happen if the concentration of acetylcholine in the synaptic cleft is very low. Low synaptic concentrations of acetylcholine can be maintained via a hydrolysis reaction catalyzed by the enzyme acetylcholinesterase. This enzyme hydrolyzes acetylcholine into acetic acid and choline. If acetylcholinesterase activity is inhibited, the synaptic concentration of acetylcholine will remain higher than normal. If this inhibition is irreversible, as in the case of exposure to many nerve gases and some pesticides, sweating, bronchial constriction, convulsions, paralysis, and possibly death can occur. Although irreversible inhibition is dangerous, beneficial effects may be derived from transient (reversible) inhibition. Drugs that inhibit acetylcholinesterase in a reversible manner have been shown to improve memory in some people with Alzheimer39。 開(kāi)放英語(yǔ)（ 2）期末自測(cè)參考答案 一部分： 1C 2C 3C 4A 5A 二部分： 6A 7C 8C 9B 10B 11C 12C 13A 14A 15C 16A 17B 18A 19C 20C 21A 22A 23C 24A 25A 三部分： 26A 27B 28C 29C 30A 31A 32B 33A 34B 35C 四部分：
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