Common Calculator Mistakes

Many students make the same calculator mistakes again and again. Here are some of the most common mistakes.

\[-3^2 =-9\]

In fact  
\[-3^2 = - 3 \times -3=9\]
People make this mistake because of the way the calculator is programmed. The simple four function calculator on or most calculators on your phone tell you  
\[-3^2 =9\]
This is correct.

Most calculators tell you  
\[20+10 \% =20.2\]
This is wrong. In fact  
\[20+10 \% =22\]
Again the simple calculator or the calculator on you pnone will give you the correct answer.

A calculator will tell you  
This is only partially correct. Every square root has two answers. In fact  
\[\sqrt{9}=3, -3\]
Many students think the solutions to  
\[cos x =0.2\]
\[cos \: 0.2\]

It is nut. Look above the cos button on youur calculator, and just above is  
\[cos^{-1} {}\]
  which is accessed using the sjift button.
The answer is  
\[cos^{-1} \: 0.2\]