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Use either the Sine or Cosine rules depending on the information you know about the triangle.
Two methods to try . #1 Use pythagoras h^ = a^2 + a^2 NB THis is only good if you know that the two unknown sides are the same length. #2 Use trigonometry (trig.) This is good if you know the hypotenuse and one of the angles. Sine(angle) = opposite/ hypotenuse Hence opposite side = hypotenuse X sine(angle) Similarly Cosine(angle) = adjacent / hypotenuse. adjacent side = hypotenuse X Cosine(angle) Here is an example If you known the hypotenuse is a length of '6' and the angle is 30 degrees. Then opposite = 6 X Sin(30) opposite = 6 x 0.5 = 3 So the length of the oppisute sides is '3' units. NB DO NOT make the mistakes of saying Sin(6 X 30) = Sin(180) Nor 6 x 30 , nor Sin(6) X 30 , nor any other combination. You MUST find the SINE of the angle , then multiply it to the given length. Similarly for Cosine and Tangent.
If you do not know only a side length you cannot. If you know all three side lengths then you can use the cosine rule. You can continue using the cosine rule for the other two angles but, once you have one angle, it is simpler to use the sine rule.
The only difference is a phase shift of pi/2 radians (90 degrees), so there is no aprticular advantage in either.
The inverse (negatives) of sine, cosine, and tangent are used to calculate the angle theta (or whatever you choose to name it). Initially it is taught that opposite over hypotenuse is equal to the sine of theta sin(theta) = opposite/hypotenuse So it can be said that theta = sin-1 (opp/hyp) This works the same way with cosine and tangent In short the inverse is simply what you use when you move the sin, cos, or tan to the other side of the equation generally to find the angle