Trig - writing an equivalent expression

**maryanna91** · Nov 14th 2009, 03:41 PM

I'm supposed to write equivalent expressions for these two expressions:

cot(sin^-1(3x))

and

sin(2sin^-1(2x))

I don't even know where to begin with these, I'm sure there's an identity I should be using to write the new expressions...but I can't figure out which one.

**DeMath** · Nov 14th 2009, 04:04 PM

Originally Posted by maryanna91

I'm supposed to write equivalent expressions for these two expressions:

cot(sin^-1(3x))

and

sin(2sin^-1(2x))

I don't even know where to begin with these, I'm sure there's an identity I should be using to write the new expressions...but I can't figure out which one.

1) $\cot \left( {{{\sin }^{ - 1}}3x} \right) = \frac{{\cos \left( {{{\sin }^{ - 1}}3x} \right)}}<br /> {{\sin \left( {{{\sin }^{ - 1}}3x} \right)}} = \frac{{\sqrt {1 - {{\left( {3x} \right)}^2}} }}<br /> {{3x}} = \frac{{\sqrt {1 - 9\,{x^2}} }}<br /> {{3x}}.$

2) $\sin \left( {2{{\sin }^{ - 1}}2x} \right) = 2\sin \left( {{{\sin }^{ - 1}}2x} \right)\cos \left( {{{\sin }^{ - 1}}2x} \right) = 2 \cdot 2x\,\sqrt {1 - {{\left( {2x} \right)}^2}} = 4x\,\sqrt {1 - 4\,{x^2}} .$

**maryanna91** · Nov 14th 2009, 04:11 PM

thanks a lot!

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