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The following four theorems are most important in solving questions on triangles.

**Pythagoras’ theorem:**In a right angle triangle the square of the hypotenuse is equal to the sum of the squares of other two sides.

${b^2} = {a^2} + {c^2}$

Pythagorean triplet: There are certain triplets which satisfy the pythagoras’ theorem and are commonly, called pythagorean triplet.

For example: 3, 4, 5; 5, 12, 13; 24, 10, 26; 24, 7, 25; 15, 8, 17

**Appolonious theorem:**

In triangle ABC, AD is median, which divides BC into two equal parts. Then,

$A{B^2} + A{C^2} = 2(A{D^2} + B{D^2}) = 2(A{D^2} + D{C^2})$

**Stewart theorem:**

In Triangle ABC, AD divides side BC in the ratio m and n. (Here AD need not be median) then,

$m.{b^2} + n.{c^2} = a({d^2} + mn)$

**Mean proportionality and Mid Point theorem:**

In the first triangle, DE // BC so $\displaystyle\frac{{AD}}{{DB}} = \frac{{AE}}{{EC}}$

In the second triangle, D and E are mid points of AB and AC respectively. Which implies, $\displaystyle\frac{{AD}}{{DB}} = \frac{{AE}}{{EC}} = 1$

Also, $DE = \displaystyle\frac{1}{2}BC$

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