Welcome back BIG — missed you this week.
Here are the solutions to your questions.
Question 1:
a) Given that (x−1) is a factor of the polynomial f(x)=ax4−x3−12x2−x+2. Find the value of the constant a and verify that f(−1)=0.
Step 1: Use the Factor Theorem.
Since (x−1) is a factor of f(x), we know that f(1)=0.
Substitute x=1 into f(x):
f(1)=a(1)4−(1)3−12(1)2−(1)+2=0
a−1−12−1+2=0
a−12=0
a=12
Step 2: Verify f(−1)=0 with a=12.
Substitute a=12 into f(x), so f(x)=12x4−x3−12x2−x+2.
Now, substitute x=−1 into f(x):
f(−1)=12(−1)4−(−1)3−12(−1)2−(−1)+2
f(−1)=12(1)−(−1)−12(1)−(−1)+2
f(−1)=12+1−12+1+2
f(−1)=4
There seems to be a discrepancy with the verification part of the question, as f(−1)=0. Assuming the question meant to ask for f(1)=0 or there's a typo in the polynomial or the verification value. Based on the given polynomial and factor, f(−1)=4.
b) Find the value of the constant k for which the equation x2+(k+1)x+k=0 has one root, double the other.
Step 1: Set up relationships between roots and coefficients.
Let the roots be α and 2α.
From Vieta's formulas:
Sum of roots: α+2α=−(k+1)⟹3α=−(k+1)(1)
Product of roots: α⋅2α=k⟹2α2=k(2)
Step 2: Solve for k.
From equation (1), α=−3k+1.
Substitute this into equation (2):
2(−3k+1)2=k
29(k+1)2=k
2(k2+2k+1)=9k
2k2+4k+2=9k
2k2−5k+2=0
Factor the quadratic equation:
2k2−4k−k+2=0
2k(k−2)−1(k−2)=0
(2k−1)(k−2)=0
So, 2k−1=0 or k−2=0.
k=21ork=2
The values of k are k=21ork=2.
Question 2:
i) Show that 1+cosθ+cos2θsinθ+sin2θ=tanθ.
Step 1: Apply double angle identities.
Recall the identities: sin2θ=2sinθcosθ and cos2θ=2cos2θ−1.
Substitute these into the left-hand side (LHS) of the equation:
LHS=1+cosθ+(2cos2θ−1)sinθ+2sinθcosθ
Step 2: Simplify the expression.
LHS=cosθ+2cos2θsinθ(1+2cosθ)
Factor out cosθ from the denominator:
LHS=cosθ(1+2cosθ)sinθ(1+2cosθ)
Cancel the common term (1+2cosθ), assuming 1+2cosθ=0:
LHS=cosθsinθ
LHS=tanθ
This matches the right-hand side (RHS).
Therefore, 1+cosθ+cos2θsinθ+sin2θ=tanθ.
ii) Find the general solution of the equation sin4x+cos2x=0.
Step 1: Use the double angle identity for sin4x.
Recall sin4x=2sin2xcos2x.
Substitute this into the equation:
2sin2xcos2x+cos2x=0
Step 2: Factor the equation.
Factor out cos2x:
cos2x(2sin2x+1)=0
This gives two separate equations:
- cos2x=0
- 2sin2x+1=0⟹sin2x=−21
Step 3: Solve for 2x in each case.
For cos2x=0:
The general solution for cosA=0 is A=2π+nπ, where n∈Z.
So, 2x=2π+nπ
x=4π+2nπ
For sin2x=−21:
The principal values for sinA=−21 are A=−6π (or 67π) and A=611π.
The general solution for $\sin