Chứng minh các đẳng thức sau: a) 2(x² + y²) = (x + y²) + ( x- y)²
b) (a + b)² - (a-b) (a+ b) = 2b ( a+ b )
c) x² + y² = ( x+ y²) - 2xy

You were told that the amount of time lapsed between consecutive trades on the new york stock exchange followed a normal distribution with a mean of 15 seconds. you were also told that the probability that the time lapsed between two consecutive trades to fall between 16 to 17 seconds was 13%. the probability that the time lapsed between two consecutive trades would fall below 13 seconds was 7%. what is the probability that the time lapsed between two consecutive trades will be between 14 and 15 seconds?