- 1). Know and understand what numbers you have, what they mean and what the formulas are for solving the angles. Let's say that you don't know any of the interior angles; all you know are the exterior sides: Side a = 3, Side b = 4 and Side c = 6.
- 2). Understand that you always need to find the largest angle first (because only one angle can be greater than 90 degrees). In this example, Side c is the largest. Use the cosine rule to find angle C: c^2 = b^2 + a^2 - 2ba(cosC)
Add 2ba(cosC) from both sides to pull it to the left of the equation and subtract c^2 from both sides to pull it to the right. Also, reverse b^2 with a^2 so that a^2 is in the front.
2ba(cosC) = a^2 + b^2 - c^2
Now divide 2ba from both sides.
cosC = a^2 + b^2 - c^2 / 2ab
Plug in your numbers and solve.
cosC = 3^2 + 4^2 - 6^2 / 2 x 3 x 4
= (3 x 3) + (4 x 4) - (6 x 6) / 2 x 3 x 4
= 9 + 16 - 36 / 24
= -11 / 24
cosC = - 0.46
Now find the inverse cos of -0.46 by using your calculator.
C = cos^-1 (-0.46)
Angle C = 117.279 degrees
- 3). Find angle A using the sides of the triangle and angle C. To find angle A, you will use the sine rule: a / sin A = c / sinC
Multiply a from both sides to solve for sinA. Also, reverse c with sinC.
sinA = a sinC / c
Plug in your numbers to solve.
sin A = 3 x sin 117.279 / 6
Input 117.279 then "sin" on your calculator.
sin A = 3 x .889 / 6
sin A = 2.667 / 6
sin A = 0.445
Determine the inverse sin of 0.445 using your calculator.
A = sin^-1 (0.445)
Angle A = 26.423 degrees
- 4). Find angle B by using the Sum of Internal Angles rule. All the internal angles have to add up to 180. The actual equation is: A + B + C = 180.
Subtract (A + C) from both sides to solve for B.
B = 180 - (A + C)
B = 180 - (26.423 + 117.279)
B = 180 - 143.702
B = 36.298
Angle B = 36.298 degrees
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