Question

1. Cho A = 1 + 3² + 3³ + .... + 3²⁰

B = 3²¹ : 2

Tinha B - A

2. Cho a = -20 ; b-c = -5 hãy tính /A/ biết

A² = b ( a -c ) - c ( a -b )

Answer 1

1. A = 1 + 3² + 3³ + ... + 3²⁰

A + 3 = 1 + 3 + 3² + 3³ + ... + 3²⁰

3(A + 3) = 3 + 3² + 3³ + ... + 3²⁰

2(A + 3) = (3 + 3² + 3³ + ... + 3²¹) - (1 + 3 + 3² + 3³ + ... + 3²⁰)

2(A + 3) = 3²¹ - 1

A + 3 = (3²¹ - 1) : 2

A + 3 = 3²¹ : 2 - \(\dfrac{1}{2}\)

A = 3²¹ : 2 - \(\dfrac{1}{2}\) - 3

A = 3²¹ : 2 - \(3\dfrac{1}{2}\)

B - A = 3²¹ : 2 - 3²¹ : 2 - \(3\dfrac{1}{2}\) = \(3\dfrac{1}{2}\)

2.

C1: A² = b(a - c) - c(a - b)

A² = ba - bc - ca + bc

A² = (ba - ca) + (bc - bc)

A² = a(b-c) + 0

A^{2 =} a(b-c)

A² = (-20).(-5)

A² = 100 \(\Rightarrow\) A = 10 hoặc A = -10 Vậy \(\left|A\right|\) = 10