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We then give the values to the hypotenuse as 13k (for some common factor k for 13 and 12) and the side opposite to angle A as 12k,

then we apply Pythagoras theorem to the triangle to find the value of the 3rd side in the triangle

we get that the value of the side adjacent to angle A is 5k.

Now we find the values of cosA and tanA

we get cosA=5/13 and tan A = 12/5

now we substitute these values in the equation to find the answer......

we get:

(144/169-25/169÷2×12/13×5/13)×5/12

=119/120×5/12= 119/288 = 0.413