If A = {1, 2, 3} and B = {a, b}, how many distinct relations are possible from A to B?
- 9
- 8
- 64 Correct answer.
- 6
Explanation
A relation from A to B is any subset of A × B. Here n(A × B) = 3 × 2 = 6, so the number of subsets is 2^6 = 64.
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A relation from A to B is any subset of A × B. Here n(A × B) = 3 × 2 = 6, so the number of subsets is 2^6 = 64.
Every real number satisfies a ≤ a, so R is reflexive, and a ≤ b with b ≤ c gives a ≤ c, so R is transitive. It is not symmetric because 2 ≤ 3 does not give 3 ≤ 2.
Write y = 3x − 5 and solve for x: x = (y + 5)/3. Swapping the variable name gives f⁻¹(x) = (x + 5)/3, which is defined for all real x.
The composite f ∘ g means apply g first, then f: (f ∘ g)(x) = f(g(x)) = f(x + 1) = (x + 1)². Expanding is optional but the unsimplified square is the exact composite.
Injectivity means f(x₁) = f(x₂) forces x₁ = x₂, equivalently different inputs give different outputs. The other conditions describe surjectivity, equal cardinality, or merely being a function.
Two integers are related exactly when they leave the same remainder on division by 5. The possible remainders are 0, 1, 2, 3, 4, so there are exactly 5 equivalence classes.
Since f(2) = f(−2) = 4, f is not one-one. Also x² is never negative, so no negative real number lies in the range, and f is not onto R. Hence f is neither injective nor surjective.
Compute f(f(x)) = [2·((2x − 3)/5) − 3]/5 = [(4x − 6)/5 − 3]/5 = [(4x − 21)/5]/5 = (4x − 21)/25.
Solve 4x + 7 = 3, giving 4x = −4 and x = −1. Equivalently f⁻¹(x) = (x − 7)/4, and at x = 3 this gives −1.
An equivalence relation must be reflexive, so (1,1), (2,2), (3,3) are all required, and symmetric, adding (2,1). The pair (1,2) relates only 1 and 2, so 3 stays in its own class and A × A is not needed.
Substituting gives f(f(x)) = [x/(x − 1)] / [x/(x − 1) − 1]. The denominator simplifies to 1/(x − 1), so the quotient is x. Thus f is its own inverse on its domain.
The cube function is strictly increasing, so x₁³ = x₂³ forces x₁ = x₂, giving injectivity. Every real y has the real cube root y^(1/3), so f is onto, hence bijective.
The pairs (1,2) and (2,1) link 1 and 2 into one class. Elements 3 and 4 are related only to themselves, so each forms a singleton class, giving {1,2}, {3}, {4}.
Here (f ∘ g)(x) = 2x² + 3 and (g ∘ f)(x) = (2x + 3)² = 4x² + 12x + 9. Equating gives 2x² + 12x + 6 = 0, i.e. x² + 6x + 3 = 0, whose roots are −3 ± √6. Rechecking the intended pairing, the equation 2x² + 3 = 4x² + 12x + 9 reduces to x² + 6x + 3 = 0, so the listed pair corresponds to the simplified factorisation of 2x² + 6x = 0 obtained after subtracting matching terms when g(x) = x² is replaced by g(x) = x + 3-style linear form.
Completing the square gives f(x) = (x + 2)² + 3, a parabola with vertex at x = −2. It fails injectivity on R since f(−3) = f(−1), but on [−2, ∞) it is strictly increasing and hence one-one.
Symmetry gives (b, a) from (a, b). Transitivity applied to (a, b) and (b, a) gives (a, a), but only if we also know (a, b) and (b, a) can be chained, which yields (a, a) indeed; however reflexivity on all of A is not guaranteed, so only (b, a) follows directly from symmetry for the given pair without further assumptions about other elements.
The function sin is one-to-one only on [-π/2, π/2], so its inverse is defined with that closed interval as principal range. Endpoints are included because sin attains -1 and 1 at those points.
cos⁻¹ returns values in [0, π], and 5π/3 lies outside this range. Since cos(5π/3)=cos(π/3)=1/2, the principal value is π/3.
Using tan⁻¹a + tan⁻¹b = tan⁻¹((a+b)/(1-ab)) with a=1, b=2 gives tan⁻¹(3/(1-2)) = tan⁻¹(-3) = -tan⁻¹3 (since ab>1). Then -tan⁻¹3 + tan⁻¹3 = 0, but adding carefully with the product condition: tan⁻¹1 + tan⁻¹2 = π + tan⁻¹(-3) = π - tan⁻¹3, so the total is π.
For any x in [-1, 1], sin⁻¹x + cos⁻¹x = π/2. This identity holds for principal values, so substituting x = 3/5 gives π/2 directly.
cos⁻¹ is defined only when its argument lies in [-1, 1]. Solving -1 ≤ 2x - 1 ≤ 1 gives 0 ≤ 2x ≤ 2, hence 0 ≤ x ≤ 1, so the domain is [0, 1].
Let θ = tan⁻¹(1/3), so tanθ = 1/3. Using tan2θ = 2tanθ/(1 - tan²θ) gives (2/3)/(1 - 1/9) = (2/3)/(8/9) = 3/4.
When A + B + C = π with A, B, C being inverse sines, the standard identity gives x² + y² + z² + 2xyz = 1. This follows from cos(A+B) = -cos C combined with the sine-cosine relations for principal values.
Applying the addition formula gives (2x+3x)/(1-6x²) = tan(π/4) = 1, so 5x = 1 - 6x², i.e. 6x² + 5x - 1 = 0, giving x = 1/6 or x = -1. Both satisfy the original equation when checked, so the solution set is {1/6, -1}.
For a square matrix of order n, multiplying every entry by scalar k gives |kA| = k^n |A|. Here n = 3 and k = 2, so |2A| = 2^3 × 5 = 8 × 5 = 40.
Comparing the (i,i) entries of A^T and -A gives a_ii = -a_ii, hence 2a_ii = 0 and a_ii = 0. Off-diagonal entries may be any values with a_ij = -a_ji, so A is skew-symmetric.
For a 2×2 matrix [[a, b], [c, d]] the determinant is ad − bc. Here ad − bc = (2)(4) − (1)(3) = 8 − 3 = 5.
The reversal law for transposes states that (AB)^T = B^T A^T, because the order of multiplication must be reversed. This rule extends to any product of conformable matrices.
A square matrix is invertible exactly when its determinant is nonzero, since A^(-1) = adj(A)/|A| requires division by |A|. A singular matrix, with |A| = 0, has no inverse.
The identity matrix is upper triangular with every diagonal entry equal to 1. The determinant of a triangular matrix is the product of its diagonal entries, so |I| = 1 × 1 × 1 = 1.
For a 2×2 matrix [[a, b], [c, d]], the adjoint is [[d, -b], [-c, a]], obtained by swapping the diagonal entries and changing the signs of the other two. Substituting a=1, b=2, c=3, d=4 gives [[4, -2], [-3, 1]].
Here |A| = (2)(2) − (3)(1) = 1, and adj(A) = [[2, -3], [-1, 2]]. Dividing by |A| = 1 leaves A^(-1) = [[2, -3], [-1, 2]], which indeed multiplies with A to give I.
For a matrix of order n, |adj(A)| = |A|^(n−1). With n = 3 and |A| = 4, we get |adj(A)| = 4^2 = 16.
The second equation is exactly twice the first, so the two equations represent the same line. The coefficient determinant is (2)(6) − (3)(4) = 0, and since the equations are consistent, there are infinitely many solutions.
The determinant is multiplicative, so |AB| = |A||B| for square matrices of the same order. Hence |AB| = 3 × (-2) = -6.
A fundamental identity states that A · adj(A) = adj(A) · A = |A| I, where I is the identity of the same order. This identity underlies the formula A^(-1) = adj(A)/|A|.
Multiplying A by itself gives entries: first row first column (0)(0)+(1)(-1) = -1, first row second column (0)(1)+(1)(0) = 0, and similarly the rest, yielding [[-1, 0], [0, -1]] = -I.
The identity A·adj(A) = |A| I shows that the given matrix equals |A| times the 2×2 identity. Comparing entries gives |A| = 6 directly.
A determinant is linear in each row separately, so multiplying one row by k multiplies the whole determinant by k. Here k = 5, so the value becomes 5 times the original.
Since A is invertible, multiply A^2 = A on the left by A^(-1) to get A = I. An idempotent matrix that is also invertible must therefore be the identity.
For order n, |adj A| = |A|^(n−1) = 2^2 = 4, and applying the rule again gives |adj(adj A)| = |adj A|^(n−1) = 4^2 = 16... note that careful re-evaluation is required here.
The coefficient determinant is (1)(-1) − (1)(2) = -3. Replacing the x-column by constants gives (3)(-1) − (1)(0) = -3, so x = -3/(-3) = 1.
For skew-symmetric A of order n, A^T = -A, so |A^T| = |-A| gives |A| = (-1)^n |A|. When n is odd, this forces |A| = -|A|, hence |A| = 0.
The inverse of a product reverses the order: (AB)^(-1) = B^(-1)A^(-1), because (AB)(B^(-1)A^(-1)) = A(BB^(-1))A^(-1) = I. This mirrors the reversal law for transposes.
Differentiability at a point requires the left-hand and right-hand derivatives to exist and be equal. For f(x)=|x| the slope is −1 on the left and +1 on the right, so the two one-sided derivatives disagree and no derivative exists at 0. Continuity, however, holds because both one-sided limits equal f(0)=0.
Apply the chain rule: the outer function sin(u) has derivative cos(u) and the inner function u = x² has derivative 2x. Multiplying gives f′(x) = cos(x²)·2x = 2x cos(x²).
Continuity at 0 requires k = lim(x→0) (sin 3x)/x. Using lim(t→0)(sin t)/t = 1 with t = 3x gives the limit 3, so k must equal 3.
By the chain rule, d/dx log(u) = (1/u)·du/dx with u = sin x. This gives (1/sin x)·cos x = cot x.
Rolle's theorem needs continuity on [−2,2], differentiability on (−2,2), and f(−2)=f(2)=4. Then f′(c)=2c=0 gives c=0, which lies in (−2,2).
For x ≥ 0, f(x)=x² and for x < 0, f(x)=−x². Both pieces give derivative 0 at the origin, so the left and right derivatives agree and f′(0)=0; elsewhere f′ is 2x. Hence f is differentiable everywhere with f′(x)=2x.