Solve the equation . cos2x−cosx=0. on the interval [0,2π)
1 answer:
Cos <span>2</span><span>x </span><span>= </span><span>2</span><span>cos^</span><span>2</span><span>(</span><span>x</span><span>) </span><span>− </span><span>1</span>
2<span>cos^</span><span>2 </span><span>(</span><span>x</span><span>) </span><span>− </span><span>1 </span><span>− </span><span>cos</span><span>(</span><span>x</span><span>) </span><span>≡ </span><span>2</span><span>cos^</span><span>2</span><span>(</span><span>x</span><span>) </span><span>− </span><span>cos</span><span>(</span><span>x</span><span>) </span><span>− </span><span>1</span>
Let <span>cos</span><span>(</span><span>x</span><span>) </span><span>= </span><span>y</span>
2<span>y^</span><span>2 </span><span>− </span><span>y </span><span>− </span><span>1
</span><span>(</span><span>2</span><span>y </span><span>+ </span><span>1</span><span>) </span><span>(</span><span>y </span><span>− </span><span>1</span><span>) </span>y <span>= </span><span>−</span><span>1/2
</span><span>y </span><span>= </span><span>1</span>
y <span>= </span><span>cos</span><span>(</span><span>x)</span>
∴ <span>cos</span><span>(</span><span>x</span><span>) </span><span>= </span><span>−</span><span>1/</span><span>2 ; </span><span>cos</span><span>(</span><span>x</span><span>) </span><span>= </span><span>1</span>
x <span>= </span><span>2/</span><span>3 </span><span>π ;</span><span> </span><span>x </span><span>= </span><span>0</span>
solutions;x <span>= </span><span>2/</span><span>3 </span><span>π</span><span>, </span><span>x </span><span>= </span><span>0</span><span>, </span><span>x </span><span>= </span><span>4/</span><span>3 </span><span>π</span><span>, </span><span>x </span><span>= </span><span>2</span><span>π</span>
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