Let be a cubic function whose leading coefficient is positive, and let be a real number. Define
so that is continuous on the entire real line and satisfies the following conditions.\begin{center}\fbox{\begin{minipage}{0.9\textwidth}(A) For every real number , the limit
exists.KOR-CSAT-(B) The set of natural numbers for which
is negative is
\end{minipage}}\end{center}Assuming that , compute . [4 points]\
[THOUGHT] Translate conditions into algebra. From (A), f(0)=f(2)=0, so f(x)=ax(x-2)*(x-r) with a>0. Continuity of g at t implies f(t)=0 so t is one of {0,2,r}. If t=r then g/(x(x-2)) becomes nonnegative a|x-r| so no negative limits, contradicting (B). If t=0 then for natural m we have g(1)=f(1)>0 making -(7/2)g(1)<0, impossible since it must be a natural number. Hence t=2. With t=2, for natural m the limit equals -a(m-r) for m<2 and a(m-r) for m>=2. To have exactly two negative m, r must be in (3,4], giving negative at m=2,3 only. Then {g(-1), -(7/2)g(1)} must equal {2,3}. Solve to find r and a, then compute g(-5). [/THOUGHT] [PYTHON] import sympy as sp
a, r = sp.symbols('a r', positive=True, real=True)
t = 2 case
g_minus1 = 3a(1+r) # derived g1 = a*(1-r) # since x=1<2 so g=-f, with f(1)=a(r-1) expr2 = -sp.Rational(7,2)*g1 # -(7/2)g(1) = (7/2)a(r-1)
Solve the only feasible assignment: g(-1)=3, expr2=2
sol = sp.solve([sp.Eq(g_minus1, 3), sp.Eq(expr2, 2)], [a, r], dict=True) print(sol)
sol = sol[0] a_val = sp.simplify(sol[a]) r_val = sp.simplify(sol[r]) print("a =", a_val, "r =", r_val)
Compute g(-5): for -5<2, g(-5) = -f(-5) = 35a(5+r)
g_minus5 = sp.simplify(35a_val(5+r_val)) print("g(-5) =", g_minus5)
Verify the set condition on natural m: compute sign for m=1..6
def L(m): m=sp.Rational(m,1) if m < 2: return sp.simplify(-a_val*(m-r_val)) else: return sp.simplify(a_val*(m-r_val)) for m in range(1,7): val=L(m) print(m, val, "negative?" , sp.sign(val)<0)
print("g(-1) =", sp.simplify(g_minus1.subs(sol)))
print("-(7/2)g(1) =", sp.simplify(expr2.subs(sol)))
[/PYTHON][THOUGHT]
Translate conditions into algebra. From (A), , so with . Continuity at forces , hence .
If , then becomes nonnegative everywhere, contradicting (B).
If , then so , impossible since it must be a natural number in (B). Thus .
With :
, , so .
The negative-limit naturals are , so . Solving gives , .
Then for , .
[THOUGHT] The Python output gives . Therefore the requested value is .