Usage of Articles(definite vs indefinte)
"Find the area of a triangle whose side is 3cm, 14cm and 7.5cm." Is this sentence correct or should we use 'the' instead of 'a' because we have specified/defined the triangle by specifying its sides(with the help of a relative pronoun)?
Use of the article 'a' is correct because when triangle is used, it hasn't been previously introduced; the specification follows after the subject.
The sentence is wrong, though, because of number: "whose side is 3cm, 14cm and 7.5cm." wrong because there are three sides (and three measurements). "whose sides are..." would be correct.