Reasoning, convincing and proving

There are 458 NRICH Mathematical resources connected to Reasoning, convincing and proving
There's a limit
problem

There's a limit

Age
14 to 18
Challenge level
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Explore the continued fraction: 2+3/(2+3/(2+3/2+...)) What do you notice when successive terms are taken? What happens to the terms if the fraction goes on indefinitely?
Diophantine n-tuples
problem

Diophantine n-tuples

Age
14 to 16
Challenge level
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Can you explain why a sequence of operations always gives you perfect squares?
Pythagorean Golden Means
problem

Pythagorean Golden Means

Age
16 to 18
Challenge level
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Show that the arithmetic mean, geometric mean and harmonic mean of a and b can be the lengths of the sides of a right-angles triangle if and only if a = bx^3, where x is the Golden Ratio.
Euler's Squares
problem

Euler's Squares

Age
14 to 16
Challenge level
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Euler found four whole numbers such that the sum of any two of the numbers is a perfect square...
Square Mean
problem

Square Mean

Age
14 to 16
Challenge level
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Is the mean of the squares of two numbers greater than, or less than, the square of their means?
N000ughty thoughts
problem

N000ughty thoughts

Age
14 to 16
Challenge level
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How many noughts are at the end of these giant numbers?
Without Calculus
problem

Without Calculus

Age
16 to 18
Challenge level
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Given that u>0 and v>0 find the smallest possible value of 1/u + 1/v given that u + v = 5 by different methods.
Cosines Rule
problem

Cosines Rule

Age
14 to 16
Challenge level
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Three points A, B and C lie in this order on a line, and P is any point in the plane. Use the Cosine Rule to prove the following statement.
Binomial
problem

Binomial

Age
16 to 18
Challenge level
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By considering powers of (1+x), show that the sum of the squares of the binomial coefficients from 0 to n is 2nCn
Triangle Incircle Iteration
problem

Triangle Incircle Iteration

Age
14 to 16
Challenge level
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Keep constructing triangles in the incircle of the previous triangle. What happens?