# How To: Derive the area of triangle using trigonometry

This video teaches the method to find the area of triangle using law of cosines and law of sines together with the area formula. Law of cosines is used when you have the length of three sides. It states that the square of side 'a' is equal to addition of the squares of sides 'b' and 'c' minus the product of 2, b, c and cosA. The values of sides are substituted and angle A is found. This is substituted in the area formula which states that the area of a triangle is equal to the half of the product of b, c, sinA. The law of sines states that the ratio of side a and sinA is equal to the ratios of b, sinB and c, sinC. So, by substituting the corresponding values the angle is found and substituted in the area formula to find the area.

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