71.

R = 500

72.

To isolate the unknown quantity in trig problems, you can use

a process similar to the "magic circle."

E = 1000

First, determine the function formula which must be used.

(Assume the formula is for the cosine function.)

Now, place the formula, cos

, in a "magic triangle",

just as we place Ohm's law in a magic circle.

And in trigonometry,

It can be seen in the triangle that cos

,that

and that x = cos θ x r.

.........................

Now, assuming the formula to be used is for the sine function,

place the formula,

in the triangle

here

.

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