= = = & gt-2 Sina & lt； 1 = = = >； Sina & gt-1/2 = = = > A belongs to (-pai/6, pai]= = >；; P=(Pai+Pai/6)/(pai+pai/3)=7/8。
12. Answer: -2. Analytic y = x (n+1) = > y' = (n+1) x n = = > y’(x = 1)= n+ 1 = = = & gt； The tangent equation is y-1= (n+1) (x-1).
Let y = 0 = = =>-1= (n+1) (xn-1) = = > xn = 1- 1/(n+ 1)= n/(n+ 1)= = = & gt； an = lgxn = lgn-LG(n+ 1)= = = & gt；
sn = LG 1-lg2+lg2-lg3+lg3-lg4+......+lgn-LG(n+ 1)=-LG(n+ 1)= = & gt；
a 1+a2+a3+......+a99=S99=-lg 100=-2。
13. Fill in: (-infinity,112 Analysis: the slope of the straight line y= 1 is 0 = > f' (x) = 0 and there is a solution. f(x)=x^3- 1/2x^2+bx+c===>； f'(x)=3x^2-x+b
= = => Equation 3x 2-x+b = 0 has a solution = = >; Discriminant =1-12b > = 0 = = = >； b & lt= 1/ 12.