tan ? = 1.2

Hmm.  Well, I suppose you could use the series expansion for tan^-1(1+x), where x would be 0.2 here.

tan^-1(1+0.2) = pi/4 + 0.2/2 - 0.2²/4 + 0.2³/12 - ...  (the angle will be in radians)

                   ≈ pi/4 + 0.1 - 0.01 + 0.0007 - ...

                   ≈ 3.1416/4 + 0.0907

                   = 0.7854 + 0.0907

                   = 0.8761 radians

But why you would want to do this beats me!

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how to do this?

To get tan^-1(1.2)  use atan(1.2)  (press 2nd atan on the calculator).

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\mathtt{1.2}}\right)} = {\mathtt{50.194\: \!428\: \!907\: \!735^{\circ}}}$$

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thankyou :D

But how to do without using the calculator?