Find out mathematical flaw...

Que: Prove that sin^4 A - sin^2 A cos^2 A +cos^4 A + 3 sin^2 A cos^2 A - (sin^2 A + cos^2 A) + sin^3 A cosec^2 A = sin A. Student: Multiply R.H.S. & L.H.S. by 0 i.e. 0=0. Teacher gave marks: 10/10 * 0 = 0. For you: Can you find out the mathematical flaw in the answer given by the student and the marks given by the teacher which have given us misleading results? NOTE: We often multiply both sides by a rational no. in such questions but if we do that with 0 it becomes misleading. What's the reason?

Note by Sarthak Kaushik
3 years, 6 months ago

MarkdownAppears as
*italics* or _italics_ italics
**bold** or __bold__ bold
- bulleted- list
• bulleted
• list
1. numbered2. list
1. numbered
2. list
Note: you must add a full line of space before and after lists for them to show up correctly
paragraph 1paragraph 2

paragraph 1

paragraph 2

[example link](https://brilliant.org)example link
> This is a quote
This is a quote
    # I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
# I indented these lines
# 4 spaces, and now they show
# up as a code block.

print "hello world"
MathAppears as
Remember to wrap math in $$...$$ or $...$ to ensure proper formatting.
2 \times 3 $$2 \times 3$$
2^{34} $$2^{34}$$
a_{i-1} $$a_{i-1}$$
\frac{2}{3} $$\frac{2}{3}$$
\sqrt{2} $$\sqrt{2}$$
\sum_{i=1}^3 $$\sum_{i=1}^3$$
\sin \theta $$\sin \theta$$
\boxed{123} $$\boxed{123}$$

Sort by:

But 0 is not -ve. Then how can you apply the concept of squaring to this problem.

- 3 years, 5 months ago

$\sin^4 A - \sin^2 A \cos^2 A +\cos^4 A + 3 \sin^2 A \cos^2 A - (\sin^2 A + \cos^2 A) + \sin^3 A \csc^2 A = \sin A$ $(\sin^2A)^2+(\cos^2A)^2+ 2 \sin^2 A \cos^2 A - 1 + \sin^3 A \csc^2 A=\sin A$ $(\sin^2A+cos^2A)^2-1+\sin^3 A \csc^2 A=\sin A$ $\sin^3A\csc^2A=\sin A$ $\sin^3A*\frac{1}{\sin^2A}=\sin A$ $\sin A=\sin A$

- 3 years, 6 months ago

I think the reason why multiplying by $$0$$ is misleading is that you cannot go back, i.e. dividing by $$0$$ is not allowed. That follows the same concept when squaring a number. For example, given a negative number, say $$-a$$ for some positive real number $$a^2$$. If you square it, it would become $$a$$ but you cannot say that getting the square root of $$a^2$$ will lead you to $$a=-a$$. I will post later for the proof and for those who cannot understand, it says: $\sin^4 A - \sin^2 A \cos^2 A +\cos^4 A + 3 \sin^2 A \cos^2 A - (\sin^2 A + \cos^2 A) + \sin^3 A \csc^2 A = \sin A$ Hint: Sum of Two Squares and Pythagorean Theorem  P.S. I thought that student was smart but then I realized that the teacher was smarter. :P

- 3 years, 6 months ago