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Usually there is an inverse key or( tan -1 )key for this
To find the tangent of 1, you can use the inverse tangent function (arctan) on a calculator. Simply input 1 into the arctan function and calculate the result. The tangent of 1 is approximately 0.7854.
To find the angle ( a ) for which ( \tan a = 0.42 ), you can use the inverse tangent function (also known as arctangent). This can be calculated using a calculator or software that provides trigonometric functions. The result will give you ( a ) in radians or degrees, depending on your calculator's settings. Specifically, ( a \approx 22.62^\circ ) or ( a \approx 0.394 ) radians.
Given only the measure of an angle, use a scientific calculator or a book of mathematical tables (if you can find one!). There are infinite series but they are not particularly easy to work with.
Nowadays you cannot do it without a calculator.