Question

[suu]

a=1.2+2.3+3.4+4.+....+200.201

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Answer 1

[Edogawa Conan]

a=1.2+2.3+3.4+4.+....+200.201

3A = 1.2.(3 - 0) + 2.3.(4 - 1) + .... + 200.201.(202 - 199)

3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + .... + 200.201.202

3A = 200.201 . 202

A = 2706800

Answer 2

[Sherlockichi Zento]

\(A=1.2+2.3+3.4+...+200.201\)

\(\frac{1}{A}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{200.201}\)

\(\frac{1}{A}=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{200}-\frac{1}{201}\)

\(\frac{1}{A}=\frac{1}{1}-\frac{1}{201}=\frac{200}{201}\)

\(A=1:\frac{200}{201}=\frac{1.201}{200}=\frac{201}{200}\)